An inductor switching DC/DC boost regulator with adaptive off/on time (AOOT) control is proposed in this paper. This system works with adaptive-off-time control in CCM while adaptive-on-time control in DCM. The burden caused by the switching frequency variation is reduced due to the quasi-fixed frequency switching by adaptive-off-time modulation under heavy-load condition. In DCM, adaptive-on-time takes over the operation and improves the power efficiency under light-load condition. In another way, an on-chip sense-FET current sensor is used to decrease the conduction loss caused by the conventional sensing resistor on power stage. The AOOT control boost regulator was implemented with a 0.25 μm high voltage (HV) CMOS process. For conversion specification of 5 V to 12V with a 0∼300mA load range, peak efficiency of 92.5% is achieved. Measurement results show that the switching frequency keeps constant at 950 kHz in CCM.