A particle image velocimetry (PIV) system has been developed to measure velocity fields with a spatial resolution of 1 micrometer. The system uses 200 nm diameter flow-tracing particles, a pulsed Nd:YAG laser, an inverted epi-fluorescent microscope, and a cooled interline-transfer CCD camera. The spatial resolution is limited by the diffraction-limited resolution of the recording optics. The accuracy of the PIV system was demonstrated by measuring the known flow field in a 30 micrometer × 300 micrometer microchannel. The resulting velocity fields have a spatial resolution of 13.6 micrometer × 0.9 micrometer × 1.8 micrometer in the streamwise, wall-normal, and out-of-plane directions, respectively. By overlapping the interrogation spots by 50% to satisfy the Nyquist sampling criterion, a velocity-vector spacing of 450 nm in the wall-normal direction is achieved. These measurements are accurate to within 2% full-scale resolution, and are the highest spatially resolved PIV measurements published to date. Recent interest in microscale devices has created a need for diagnostic tools with spatial resolutions on the order of several microns. PIV is a well-established technique for measuring velocity fields in macroscopic fluid systems. The first successful micro-PIV experiment used an epi-fluorescent microscope and a Princeton Instruments' intensified CCD camera to record the flow around a 30 micrometer diacylinder. The flow-tracing particles were 300 nm diameter polystyrene particles. These particles were large enough to emit sufficient light for recording and to reduce the effects of Brownian motion. In this paper, an alternative approach is described where a 5 ns pulsed Nd:YAG laser is used to illuminate sub-micron fluorescent particles. The fluorescent images are recorded using a cooled interline transfer CCD camera that is capable of taking back-to-back images within a time interval as short as 500 ns. The CCD camera's pixel spacing is 6.8 micrometer. The required density of flow-tracing particles in micro PIV experiments is commonly on the same order as that for macro PIV experiments. The required density of flow-tracing particles in micro PIV experiments is commonly on the same order as that for macro PIV experiments. The required density of flow-tracing particles in micro PIV experiments is commonly on the same order as that for macro PIV experiments.A particle image velocimetry (PIV) system has been developed to measure velocity fields with a spatial resolution of 1 micrometer. The system uses 200 nm diameter flow-tracing particles, a pulsed Nd:YAG laser, an inverted epi-fluorescent microscope, and a cooled interline-transfer CCD camera. The spatial resolution is limited by the diffraction-limited resolution of the recording optics. The accuracy of the PIV system was demonstrated by measuring the known flow field in a 30 micrometer × 300 micrometer microchannel. The resulting velocity fields have a spatial resolution of 13.6 micrometer × 0.9 micrometer × 1.8 micrometer in the streamwise, wall-normal, and out-of-plane directions, respectively. By overlapping the interrogation spots by 50% to satisfy the Nyquist sampling criterion, a velocity-vector spacing of 450 nm in the wall-normal direction is achieved. These measurements are accurate to within 2% full-scale resolution, and are the highest spatially resolved PIV measurements published to date. Recent interest in microscale devices has created a need for diagnostic tools with spatial resolutions on the order of several microns. PIV is a well-established technique for measuring velocity fields in macroscopic fluid systems. The first successful micro-PIV experiment used an epi-fluorescent microscope and a Princeton Instruments' intensified CCD camera to record the flow around a 30 micrometer diacylinder. The flow-tracing particles were 300 nm diameter polystyrene particles. These particles were large enough to emit sufficient light for recording and to reduce the effects of Brownian motion. In this paper, an alternative approach is described where a 5 ns pulsed Nd:YAG laser is used to illuminate sub-micron fluorescent particles. The fluorescent images are recorded using a cooled interline transfer CCD camera that is capable of taking back-to-back images within a time interval as short as 500 ns. The CCD camera's pixel spacing is 6.8 micrometer. The required density of flow-tracing particles in micro PIV experiments is commonly on the same order as that for macro PIV experiments. The required density of flow-tracing particles in micro PIV experiments is commonly on the same order as that for macro PIV experiments. The required density of flow-tracing particles in micro PIV experiments is commonly on the same order as that for macro PIV experiments.