One of the important problems in the development of micro grid is related with its operation optimization including multiple distributed generators(DG). To improve the operation level of the micro grid, the modified particle swarm optimization(PSO)algorithm based on chaos theory and simulated annealing algorithm is proposed in the paper. The mathematical model regarding optimal operation of micro grid is established incorporating different operation indexes of the distributed generators, in which the optimized object function is assumed to be correlated with operation indexes such as operation cost and environmental benefit. The modified PSO algorithm is used to solve the multi-objective problem, with the optimized operation of the mirco-grid with DGs being obtained. A case study regarding 24 hours operation of one mico-grid shows that, the modified PSO algorithm appears to have enhanced optimization ability and convergence rate.