We have synthesized un-doped and Co-doped ZnO nanoparticles (NPs) by precipitation method at a temperature of 400 ℃ for 2 hours. According to the inorganic crystal structure database (ICSD) number #98-005-7478, the XRD pattern of ZnO NPs (average diameter of 99-144 nm) possess polycrystalline hexagonal wurtzite structure. The crystallite size of un-doped and 1.75 at.%, 3.79 at.%, 4.37 at.% Co-doped ZnO NPs are 20.81 nm, 19.84 nm, 43.85 nm, 37.95 nm, respectively. Meanwhile, the average micro-strain of the un-doped ZnO NPs and 1.75 at.%, 3.79 at.%, 4.37 at.% Co-doped ZnO NPs are 0.62%, 0.66%, 0.29%, 0.34%. These results confirm the crystallite size and micro-strain changing by Co incorporation. Further investigation, the Co incorporation into Zn site improve absorbance of ZnO NPs. Meanwhile, photoluminescence (PL) measurement shows that all ZnO NPs have one broad emission with centered peaks of 385 nm. It is indicating the substitution Zn²⁺ by Co²⁺ cause the distribution defect spread continuously in ZnO NPs structure

Keywords: un-doped and Co-doped ZnO NPs, polycrystalline, crystallite size, micro-strain, photoluminescence