Co2+ & Er3+ co-doped ZnO nanocrystals were synthesized by the hydrothermal method at 180°C and pH= 12 for 48 h. Powder XRD patterns indicate that the Zn1-2xErxCoxO crystals (0.00<x≤0.035) are isostructural with ZnO. The cell parameters increase for Er3+ and Co2+ upon increasing the dopant content (x). SEM images show that doping of Er3+ and Co2+ into the sites of Zn2+ does not change the morphology of ZnO. The FT-IR results indicate that the change in the peak position of ZnO absorption bands reflect that Zn-O-Zn network is perturbed by the presence of Er and Co in its environment. Semiconductor properties of these oxides are investigated by UV-Vis spectroscopy that there are red shifts as well as decreasing intensity of absorption peaks in doped nanomaterials.