Process Nanosize Ni\-xMn\-\{3-x\}O\-4(0.5≤x≤1) powder was prepared by microwave heating in a multi mode cavity of 2.45GHz.The experimental results showed that Ni\-xMn\-\{3-x\}O\-4(0.5≤x≤1) phase structure was all spinel type at 600℃ and the grains of samples prepared by microwave heating are small and uniform.The synthetic temperature of Ni\-xMn\-\{3-x\}O\-4(0.5≤x≤1) spinel phase by microwave heating is 200℃ lower than sol gel method and conventional method.