The development of machine vision technology provides an effective method for automatic measurement of particle size distribution. However,the overlapping particles in theimageis still difficult to be segmented. To solve this problem, one kind ofpellet image segmentation algorithm based on morphological reconstruction and Gauss mixture model is proposed. To achieve unsupervised clustering, morphological reconstruction combined with clustering validity index isused to obtain the optimal number of clusters.EM algorithm isutilizedto solve this problem. Finally, the missing particle contours arereconstructed by circle fitting method.The segmentation of overlapping pellets is realized. Experimental results show that the algorithm can effectively segment overlapping pellets.The segmentation accuracy evaluation index AC is 936%, which is obviously superior to the compared algorithms. The measurement of particle size distribution based on machine vision is founded.