A high selectivity wideband balun bandpass filter based on substrate integrated waveguide (SIW) and complementary split rings resonators (CSRRs) is proposed. 180 • reverse phase characteristic between the two output ports can be easily realized by the multi-layer SIW power divider. Eight complementary split rings resonators are used to achieve the sharp rejection upper stopband. The proposed wideband balun filter exhibits a fractional bandwidth of 37% centered at 9.45 GHz and amplitude and phase imbalance less than 0.5 dB and 1 • .