This paper presents a new application of absorptive bandstop filter for absorbing the LO-to-RF leakage of mixer in a Q-band heterodyne receiver. The proposed absorptive bandstop filter (ABSF) is achieved by introducing lossy quarter-wavelength resonator to a conventional narrowband bandstop filter. In order to facilitate the receiver system integration, the proposed ABSF is implemented in commercial GaAs pHEMT process. It is designed with a stopband from 31 to 33 GHz, which is the LO frequency range. The measured stopband rejection is better than 35 dB at the stopband center frequency f0 = 32 GHz. Especially, more than 93.4% of the power within the stopband can be dissipated by the ABSF. In this way, by introducing the proposed ABSF between the image rejection filter and the mixer, the impact of LO-to-RF leakage on the conversion gain of mixer can be minimized.