In this paper, we propose a differentially fed ultrawideband (UWB) quasi-self-complementary (QSC) antenna with a 5.5-GHz notch band. By designing the first five modes of the complementary antenna in the passband, the desired UWB response is achieved. To suppress the common-mode (CM) signals, instead of directly connecting to the complementary antenna, the differential feedline is deployed symmetrically across the input slotline so that weak coupling can be established when in CM operation. A 5.5-GHz notch band for preventing undesired WLAN signals is implemented by tapping λ/4 open stubs near the bisection plane. The differentially fed UWB complementary antenna have good differential-mode (DM) performance, high CM rejection, as well as low DM-to-CM conversion. The measured results show that the 10-dB DM passband is from 2.75 to 11.01 GHz, except the notch band; the CM reflection coefficient is greater than-2.62 dB and the DM-to-CM conversion is lower than-24.7 dB in the 2-12 GHz frequency range.