Abstract: A metallic metamaterial high power microwave (HPM) mode converter which converts cylindrical TEM mode to TE11 mode is presented. Metallic sector grid elements are periodically arranged both in the azimuthally and longitudinal direction to gain high refractive index. High refractive index metamaterial fills half space of the cylindrical transmission line of the mode converter in the azimuthally direction. The phase of electric field was shifted from 0 degree to 180 degrees between the metamaterial part and the vacuum part as the microwave propagates though the mode converter, hence the microwave mode is converted from TEM mode to TE11 mode. The mode converter is simulated by 3D commercial electromagnetic simulation software CST Microwave Studio. Simulation results show that the converter has high convert efficiency with bandwidth of about 4%. Radiation antenna with metamaterial mode converter is simulated. The radiation pattern reveals a typical TE11 mode pattern which validates the design of mode converter.