In order to measure the solar irradiance and the earth radiation, a Solar Irradiance Absolute Radiometer (SIAR 1) is constructed. Two important improvements are made on electro calibrated cavity absolute radiometer and the absolute accuracy of SIAR 1 is improved. SIAR 1 has completed the international comparison with WRR in WRC, Switzerland during Sept. 25 to Oct. 13 in 2000, the comparison results showed that SIAR 1 is 0.08% higher than WRR. Whereafter, comparison of STIM with SIAR 1 is made. The average standard deviation among three absolute radiometers of STIM is 0.11%, The second absolute radiometer is accordance with SIAR 1 in the range of 0.09%. So it is concluded that the absolute accuracy of STIM is 0.16%.