An anomalous character of reflectivity of resonance active medium is analysed theoretically based on the treatment of nonlinear optical susceptibility of two level atomic system. The calculation curves of the relationship between B and A A* and B and 5 (A is the amplitude of the electric field of the laser beam, while 5 is the dimensionless detuning of center line of the atomic system about the’center frequency e0)) are obtained for the laser active medium as Nd: YAG crystal. It is shown that there is still dependence of B on A A* even at very low intensity of AA4.*-10-4.