A new method for measurement of the laser transition cross-section and fluorescence lifetime is introduced. It is based on the relation between the laser relaxation oscillation frequency and the laser parameters. By this method, the transition cross-section and the fluorescence lifetime of Nd:YAG laser at 1.0642μm and Nd:YAP laser at 1.0795μm are measured to be 43×10-20 cm2, 252μs and 46×10-20cm2, 142μs respectively.