ZnO has been reported to be a promising photocatalytic material for dye degradation. But the quick recombination of the photo-generated charge carriers and low stability due to photo corrosion has been a major drawback of these material. In addition to this, the large band gap of ZnO makes it inefficient for the utilization of Sun light, specially in the visible region. In order to overcome these demerits, various methods like modification of morphology, doping, forming composite, etc. have been adopted. This work reports the effect of Cu ion doping in ZnO nanomaterials on its photocatalytic characteristics. Pure and Cu ion doped ZnO (Cu:ZnO) nanopowders with varying Cu ion content from 0 to 20 wt% had been synthesized by hydrothermal method. Single phase of wurtzite hexagonal ZnO was formed up to 10 wt% of Cu doping into the ZnO, beyond which the samples have become biphasic with the introduction of CuO as secondary phase, showing that 10 wt% was the solubility limit of Cu in ZnO lattice. Average crystallite size calculated by Scherrer formula was in the range from 21.9 to 39.0 nm. Strain in pure and Cu:ZnO was calculated by Williamson and Hall analysis. Optical absorption analysis showed that the band gap had decreased from 3.25 to 3.11 eV with increasing Cu content. Near-band edge (NBE) and a broad yellow emission bands with the maxima respectively at 382 and 561 nm were observed in the photoluminescence spectra of Cu:ZnO. The photocatalytic activity of these samples was investigated by the oxidation of methyl orange dye under UV light as well as under Sun light irradiation. The results indicated that the photocatalytic degradation of the MO solution under UV irradiation shows 83.8% for ZnO whereas the 1% Cu:ZnO have degraded the best with 99.5% of MO under Sun light irradiation.