This paper presents a method of thermal balancing for monolithic power integrated circuits (ICs). An on-chip temperature monitoring sensor that consists of a poly resistor strip in each of multiple parallel MOSFET banks is developed. A temperature-to-frequency converter (TFC) is proposed to quantize on-chip temperature. A pulse-width-modulation (PWM) methodology is developed to balance the channel temperature based on the quantization. The modulated PWM pulses control the hottest of metal-oxide-semiconductor field-effect transistor (MOSFET) bank to reduce its power dissipation and heat generation. A test chip with eight parallel MOSFET banks is fabricated in TSMC 0.25 µm HV BCD processes, and total area is 900 × 914 µm2. The maximal temperature variation among the eight banks can reduce to 2.8°C by the proposed thermal balancing system from 9.5°C with 1.5 W dissipation. As a result, our proposed system improves the lifetime of a power MOSFET by 20%.