By means of microwave (2. 45 GHz) exciting high pure oxygen to generate O2(1△g) dimer, the gain coefficients of O2 (2△g) dimer at 486 um was obtained for the first time by the method of amplified spontaneous emission. The laser oscillation of the emission at 486 nm was observed with a flat-concave stable resonator. The maximum laser power measured was 130 μW and the divergent angle was 28 mrad.