Optical and electrical parameters of high-power LED at linear and pulse widt h modulated (PWM) dimming modes are tested at the same input electric power.C omparing color temperatures,luminous efficienies and radiant efficiencies at different input electric power levels,it is observed that at two dimming mod es,the color temperature decreases with decreasing average input current,and it drifts greater at linear dimming mode than PWM dimming mode.In addition,the l uminous efficiency and radiant efficiency decrease with increasing average input mode than that at PWM dimming mode.Moreover,with decreasing average input cu rrent,the radiant efficiency curve goes faster at linear dimming mode than th at at PWM dimming mode.The thermal transient tester is used to build LED tran sient thermal impedance model.The phenomenon that the radiant efficiency of LED varies with junction temperature at linear and PWM dimming modes is analyzed using the electric-thermal simulation method.It is concluded that at PWM dim ming mode,a decrease of radiant efficiency is mainly affected by junction tempe rature.However,at linear dimming mode,a decrease of radiant efficiency is not only affected by junction temperature,but also by invalid injection current.