Abstract: An optimal evolution algorithm for image thresholding is proposed based on theories of genetic algorithm.Pixel and threshold are regarded as chromosome and evolution direction,respectively.Assuming the optimal evolution direction exists,the updating model of evolution direction is established.Then by defining the chromosomes' coding rules,initializing the group by simple-random-sampling,and redefining the fitness function and the selection mechanism,the optimal threshold is obtained under the proper crossover rate and mutation rate.The rationalities of the assumption and the updating model have been analyzed in this paper.The experimental results show that the assumption and the updating model are proper;the proposed algorithm is robust and effective.