Abstract: A wideband semi-circular Substrate Integrated Waveguide (SIW) antenna with complementary E-shaped radiators is presented. In order to obtain a wide operation bandwidth, the main body of the SIW antenna is designed as a semi-circle angled with the input microstrip. For further expansion of the impedance bandwidth, the complementary slots and microstrip lines are designed as E-shape, yielding high radiation gain over the operation band. To improve the impedance matching of the proposed antenna, a metallic via is added and optimized. Experimental results demonstrate that the proposed antenna yields a -10 dB impedance bandwidth over the band of 37.7~47.8 GHz. Moreover, the realized gains at all frequencies above 6.4 dBi can be achieved over the bandwidth of operation, with a maximum of 9.8 dBi at 38.2 GHz. Wideband and high gain are achieved simultaneously.