ZnO-ZnMgO-MgO nanocomposites have been successfully synthesized using the sol-gel method. An enlargement, with Mg content, of the XRD band relative to (002) diffraction revealed the formation of ZnMgO phase. HRTEM images prove the formation of ZnO and MgO nanocrystals. Raman spectrum show a weakened surface vibration and broadened LO peak with high Mg concentration. UV-vis measurements demonstrate the appearance of double band gap relatives to ZnO and ZnMgO phases. A high sensitivity of the UV emission band to Mg content was detected through the PL analysis. The photocatalytic activity of the samples was evaluated for aqueous Rhodamine B solution under sunlight irradiation. ZnO-ZnMgO-MgO nanocomposites with of 20% of Mg content display the best activity with a full degradation after 80 min of irradiance. The present work could provide an easy and low-cost way for the fabrication, high efficiency of ZnO-MgO photocatalyst with the appearance of the ternary alloy ZnMgO phase.