This study presents a method for estimating lattice strain in ZnO nanoparticles using X-ray peak profile analysis. ZnO nanoparticles were synthesized using a precipitation method from chitosan and zinc chloride. The synthesized nanoparticles were characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL). The XRD results showed that the sample had a hexagonal wurtzite structure. The crystallite size and lattice strain were analyzed using the Williamson-Hall (W-H) method and size-strain plot (SSP). The results showed that the crystallite size estimated from Scherrer's formula, W-H plots, and SSP were highly correlated. The crystallite size and particle size obtained from XRD and TEM were found to be very similar. The lattice strain was calculated using the W-H and SSP methods, and the results were found to be comparable. The SSP method was found to be the most suitable for estimating lattice strain and crystallite size. The study also showed that the lattice strain in ZnO nanoparticles was due to the small crystallite size and lattice strain. The results indicated that the ZnO nanoparticles had a hexagonal wurtzite structure with a crystallite size of about 27.49 nm. The lattice strain was found to be 14.76 × 10⁻⁴ nm⁻². The study also showed that the ZnO nanoparticles had a high surface area and a large number of surface states related to oxygen vacancies. The PL spectrum showed a narrow and weak UV emission band at 390 nm and a stronger and broader visible emission band at 470 nm. The study concluded that the ZnO nanoparticles had a hexagonal wurtzite structure with a crystallite size of about 27.49 nm and a lattice strain of 14.76 × 10⁻⁴ nm⁻². The results indicated that the ZnO nanoparticles had a high purity and excellent yield, making them suitable for various applications.This study presents a method for estimating lattice strain in ZnO nanoparticles using X-ray peak profile analysis. ZnO nanoparticles were synthesized using a precipitation method from chitosan and zinc chloride. The synthesized nanoparticles were characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL). The XRD results showed that the sample had a hexagonal wurtzite structure. The crystallite size and lattice strain were analyzed using the Williamson-Hall (W-H) method and size-strain plot (SSP). The results showed that the crystallite size estimated from Scherrer's formula, W-H plots, and SSP were highly correlated. The crystallite size and particle size obtained from XRD and TEM were found to be very similar. The lattice strain was calculated using the W-H and SSP methods, and the results were found to be comparable. The SSP method was found to be the most suitable for estimating lattice strain and crystallite size. The study also showed that the lattice strain in ZnO nanoparticles was due to the small crystallite size and lattice strain. The results indicated that the ZnO nanoparticles had a hexagonal wurtzite structure with a crystallite size of about 27.49 nm. The lattice strain was found to be 14.76 × 10⁻⁴ nm⁻². The study also showed that the ZnO nanoparticles had a high surface area and a large number of surface states related to oxygen vacancies. The PL spectrum showed a narrow and weak UV emission band at 390 nm and a stronger and broader visible emission band at 470 nm. The study concluded that the ZnO nanoparticles had a hexagonal wurtzite structure with a crystallite size of about 27.49 nm and a lattice strain of 14.76 × 10⁻⁴ nm⁻². The results indicated that the ZnO nanoparticles had a high purity and excellent yield, making them suitable for various applications.