This paper presents a sub-harmonic mixer IC design for W-band automotive radar applications in 130-nm SiGe BiCMOS technology. The mixer makes use of a Common Emitter Common Collector Transistor Pair (CECCTP) structure mixer core with a Marchand balun for the W-band on-wafer measurement. The balun achieves a measured amplitude imbalanced of less than 0.9 dB and a phase imbalance of less than 2.5 degrees in a frequency range from 20 GHz to 66 GHz. The sub-harmonic mixer IC exhibits a conversion gain of 3.8 dB at 80 GHz with an LO power of 0 dBm at 39.5 GHz. And the mixer core only consumes 0.42 mA with a supply voltage of 2.5 V.