A circuit and a method for restarting up a VCO of a PLL are introduced herein. The VCO restart up circuit receives a power down signal, an external signal, a clock output from the VCO and generates a trigger signal to the VCO to trigger the VCO clock to leave a stable mode. In other words, if the VCO clock is in the stable mode, the VCO restart up circuit generates one or more than one pulse on a trigger signal to restart up the VCO. Oppositely, if the VCO is not in the stable mode, there is no pulse on the trigger signal generated by the VCO restart up circuit and the VCO needs not to be restarted up.