A bootstrap circuit is utilized in a bulk circuit using an NMOS transistor as a power switch. The bootstrap circuit includes a first PMOS transistor coupled between an internal power source and an offset capacitor, and a second PMOS transistor coupled between the gate of the first PMOS transistor and the offset capacitor, and an NMOS transistor coupled between the gate of the first PMOS transistor and ground. When the power switch is turned on, the second PMOS transistor is turned on for turning off the first PMOS transistor. When the power switch is turned off, the NMOS transistor is turned on for turning on the first PMOS transistor.