In this study, we compare the structural properties of the three phases wurtzite (B4), zinc blende (B3), and rocksalt (B1) of ZnO using the generalized gradient approximation (GGA) and the modified Becke-Johnson TB-mBJ potential to compare their electronic and optical properties. It's found that the wurtzite phase is energetically the most stable. Zinc blende and wurtzite structures show similar electronic and optical properties. These results are compared to experimental data and previous theoretical works and they were found to be in good agreement.