A level shifter includes a Not gate coupled to a signal input and operable between a first high level and a low level; a first PMOS transistor coupled to a second voltage source and a control end; a first NMOS transistor coupled to the first PMOS transistor, a Not-gate output end and a reference voltage; and a control circuit coupled to the signal input, the Not-gate output end and the second voltage source. When the signal input and the Not-gate output end are at the first high level and the low level, respectively, the first PMOS transistor is turned on so that the signal output is at a second high level; and when the signal input and the Not-gate output end are switched contrarily, the first PMOS transistor is turned off and the signal output is at the low level.