Abstract: For well understanding of light propagation in photonic crystal micro-cavity,the transmission spectrum of a one-dimensional photonic crystal contained Lorentz dispersive medium was calculated by means of the finite-difference time-domain method.The influence of refractive index inside micro-cavity on amplification properties and resonant frequency of defect modes was analyzed in detail.The results show that dispersive properties inside the micro-cavity lead to frequency shift of the defect modes.In addition,these defect modes can be also amplified as optical gain is introduced into the system.It is shown that lasing threshold of the defect modes can be significantly reduced through carefully adjusting the refractive index of micro-cavity.