Nonlinearity of photoelectric pyrometer is one of the main uncertainty sources for realization of international temperature scale.A novel nonlinearity measurement system based on light-emitting diode (LED) light sources and flux addition method is established.A high-brightness LED with nominal 645 nm peak wavelength is utilized as light source to substitute conventional tungsten lamp.The switch of power supply for LED can realize flux addition,and the abandonment of optical strobe can avoid the nonideal result from repeated reflection.The experimental research on the spectral radiation drift of LED under step-mode power supply is carried out to validate the end time of nonlinear drift stage.The pulse-mode power supply of fixed phase shift for double LEDs and beam splitter are combined to attain flux addition.The linear drift stage of LED is chosen to measure the nonlinearity of pyrometer.The linear drift effect on the nonlinearity measurement is eliminated by exchange the measurement turn and experiments are carried out to validate the method.The nonlinearity of the standard radiation thermometer (model RT9032) is experimentally studied.