This paper presents a compact equivalent lumped circuit model for on-chip helical transformers. From fundamental model, a simple coupled RL loop is added to account for the skin and proximity effect, and a series RLC branch is added to account for the parasitic coupling effects between inductors. Several helical transformers with various design configurations were fabricated using a standard 0.18 µm 1P6M CMOS process. It’s demonstrated that the proposed model shows good accuracy over two times self-resonance frequency, which is essential for the design of high-speed and radio-frequency circuit.