This paper present an ultra wide band 180° power divider based on transition between microstrip and slot line to achieve ultra wideband 3 dB power distribution. In order to expand the bandwidth of the power divider, the fan shaped transition structure is used in this workin instead of the traditional round transition structure. The simulation results and the measurement results are agreed well. The insertion loss of less than 1.5 dB at the frequency range of 3.1~10.6 GHz has been obtained.The amplitude balance is less than 0.8 dB and the phase balance is less than 1° between the two output ports;the reflection coefficient at the input port is less than -12 dB.