In this paper, an unsupervised change detection method based on space contextual information and EM algorithm is proposed. In the algorithm, each pixel of the difference image is represented by a characteristic quantity constructed from the difference image values considering the space contextual information. EM algorithm is used to achieve the parameter estimation of each class pixels. Bayesian inference is then employed to perform the final change detection results. Experimental results obtained on multi-temporal optical images acquired by Landsat 5 TM confirm the effectiveness of the proposed approach.