The present invention provides a low-cost resonant inverter circuit for ballast. The resonant circuit includes a transformer connected in series with a lamp to operate the lamp. A first transistor and a second transistor are coupled to switch the resonant inverter circuit. A second winding and a third winding of the transformer are used for generating control signals in response to a switching current of the resonant inverter circuit. The transistor is turned on once the control signal is higher than a high-threshold. Next, the transistor is turned off once the control signal is lower than a low-threshold. Therefore, soft switching operation for the first transistor and the second transistor is achieved.