An inverter capable of switching working frequency for activating a lamp. The inverter comprises a voltage converter, a feedback circuit, a control unit and a frequency switcher. The voltage converter transforms a DC input voltage into a second AC voltage according to a control signal. The lamp is activated and generates a feedback current. The circuit converts the feedback current into the feedback voltage. The control unit for generating a first working frequency includes a resonance circuit having a specific resistance and capacitance, and a control device for generating the control signal according to a reference voltage, the first working frequency and the feedback voltage in a predetermined period. Then, the switcher changes the specific resistance and/or capacitance for enabling the control unit to generate a second working frequency. The control device outputs the control signal according to the second working frequency and the feedback voltage.