A high efficiency buck converter has power-saving means to improve the efficiency under both heavy load and light load conditions. A first circuit generates an oscillation signal and a power-saving signal in response to a feedback signal. In reference to the feedback signal and the oscillation signal, a second circuit generates switching signals to control switching devices. An off-time of the switching signal increases in response to a decrement of load. The power-saving signal turns off the switching devices and parts of control circuits of the buck converter during the off-time of the switching signal for saving power under light load conditions.