The gain coefficients of dimer of O2(1Δg) was estimated theoretically. Using microwave to generate singlet oxygen (1Δg) in our experiments, the gain characteristic of the dimer of O2(1Δg) at 6561 A has been obtained for the first times by the means of amplified spontaneous emission. When bho pressure was 0.3torr and the power of the microwave was 300 W, the net gain was 1.83% cm-1. It is expected to laser osoialla-tion in the medium.