This paper presents a compact 35-65 GHz Gilbert cell up-convert mixer implemented in TSMC 0.18μm SiGe BiCMOS technology. Integrated broadband transformers and meandered thin-film microstrip lines were utilized to achieve a miniature chip area of 0.6 mm × 0.45 mm. The compact MMIC has a flat measured conversion loss of 7 ± 1.5 dB and LO suppression of more than 40 dB at the RF port from 35 to 65 GHz. The power consumption is 14 mW from a 4-V supply. This is a fully integrated millimeter-wave active mixer that has the smallest chip area ever reported, and also the highest operation frequency among up-conversion mixers using silicon-based technology.