The medium frequency (400 Hz) power systems supply energy to the critical system and loads in aerospace, airplane and vessel, where space and weight are at a premium. The converters are demanded to obtain 400Hz power from civil 50/60Hz power systems. There are mainly two types of converters: solid state frequency converter (SSFC) and rotary frequency converter (RFC). There are various evaluations about the converters in different papers. The eight characteristics of two converters have been investigated in detail. The comparison between solid state frequency converters and rotary frequency converters is analyzed, and the applicable requirements and conclusions are presented in the final.