A compact size millimeter-wave on-chip defected ground structure (DGS)-based bandstop filter (BSF) is proposed and implemented in 0.18-μm CMOS technology. The proposed BSF is realized with T-shape DGS. In combination with the series resonance circuits, the proposed design realizes two transmission poles: one at either side of parallel resonance. Despite the compact size, the proposed BSF still maintains higher quality (Q) factors and group delay. The proposed BSF optimized at 52 GHz frequency is fabricated and tested. The active area of the fabricated prototype without pads is 0.0129 mm2 which is only 50% of the size of a wideband BSF recently proposed in a similar technology  and . The measured loaded and unloaded Q-factors are 4.57 and 20.55, respectively.