Microgrids， which include various distributed energy resources and controllable loads， can be operated and dispatched flexibly. They could operate in both grid-connected and stand-alone modes. Based on steady-state characteristics of distributed energy resources， an optimization and dispatch model for microgrids in the centralized control mode is developed. The objective functions with minimal cost of operation or depreciation， and maximal the environmental or comprehensive benefit are discussed. The optimization and dispatch model is validated by studying a specified case with different interactive operational control strategies， objective functions and power prices.