A highly efficient tri-mode buck converter with a novel load sensor circuit is proposed in this paper. The combination of pulse-width modulation (PWM), pulse-frequency modulation (PFM) and pulse-skipping circuit makes it efficient to operate in wide loading region. By using the novel load sensor, the buck converter can achieve high efficiency for the load range from 3mA to 400mA. This load sensor can automatically switch the modes according to the different load conditions. It changes the modulation methods from PWM mode to pulse-skipping modulation mode or to PFM mode. This tri-mode buck converter is fabricated by TSMC 0.35um. The efficiency above 90% is achieved for the load range about 90-400mA in PWM operation, and above 78% attained for the load range about 3-50mA in PFM mode. The insertion of the pulse skipping mode operation extends the high efficiency 90% of PWM operation for the load range from 90mA down to 50mA.