A 79-GHz band miniature mixer fabricated by using our 3-D WLCSP technology and PHEMT is demonstrated. The mixer consists of a balanced harmonic mixer pair to enhance LO-to-RF leakage suppression. The balun for out-of-phase power splitting is a half-sized Marchand balun, effectively miniaturizing the mixer unit. The 40-GHz and 80-GHz band baluns were designed in the same fashion. Though their bandwidth becomes much narrower than the popular full-size one, the realized bandwidth and 180-degree phase difference were good enough for the radar application. The achieved balanced harmonic mixer is being successfully applied to RX and TX for intelligent automotive radar system.