This study investigates the synthesis and characterization of ZnO nanoparticles using a precipitation method from chitosan and zinc chloride. The synthesized nanoparticles were characterized using various techniques, including Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL). XRD analysis revealed a hexagonal wurtzite phase, confirming the crystalline nature of the nanoparticles. The crystallite size and lattice strain were estimated using X-ray peak profile analysis (XPPA) and modified Williamson–Hall (W–H) plots. The Scherrer method, W–H models (uniform deformation model, uniform stress deformation model, and uniform deformation energy density model), and size–strain plot (SSP) were employed to determine the crystallite size and lattice strain. The results showed that the crystallite size and particle size estimated from different methods were highly correlated, with both XRD and TEM providing consistent results. The PL spectrum exhibited a weak UV emission band at 390 nm and a stronger visible emission band at 470 nm, indicating the presence of surface states related to oxygen vacancies. The study highlights the importance of SSP and W–H models in determining the crystallite size and lattice strain of ZnO nanomaterials, which are crucial for understanding their structural properties and potential applications.This study investigates the synthesis and characterization of ZnO nanoparticles using a precipitation method from chitosan and zinc chloride. The synthesized nanoparticles were characterized using various techniques, including Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL). XRD analysis revealed a hexagonal wurtzite phase, confirming the crystalline nature of the nanoparticles. The crystallite size and lattice strain were estimated using X-ray peak profile analysis (XPPA) and modified Williamson–Hall (W–H) plots. The Scherrer method, W–H models (uniform deformation model, uniform stress deformation model, and uniform deformation energy density model), and size–strain plot (SSP) were employed to determine the crystallite size and lattice strain. The results showed that the crystallite size and particle size estimated from different methods were highly correlated, with both XRD and TEM providing consistent results. The PL spectrum exhibited a weak UV emission band at 390 nm and a stronger visible emission band at 470 nm, indicating the presence of surface states related to oxygen vacancies. The study highlights the importance of SSP and W–H models in determining the crystallite size and lattice strain of ZnO nanomaterials, which are crucial for understanding their structural properties and potential applications.