In order to solve the partition problem of coherent generators in the dynamic equivalent method based oncoherency,an algorithm marked as PSO-FCM,namely,the fuzzy c-means clustering ( FCM) based on the particleswarm optimization ( PSO) ,is presented.In this algorithm,the cluster center is taken as a particle for encoding,and the parallel property and the global searching capacity of the PSO algorithm are applied to the optimizationthrough continuously renewing the velocity and position of the particle. Thus,the dependency of the FCM algorithmupon the initial value and the risk of falling into local optimization are overcome. Moreover,in order to evaluate theclustering validity,a validity function is constructed. Simulated results on IEEE 10-generator 39-bus system showthat the PSO-FCM algorithm is suitable for the partition of coherent generators of electric power systems in variousoperation modes because it is of simplicity,easy realization,high accuracy and high speed.