According to the policy implementation of energy-saving and emission-reduction, and the situation of renewable energy merged into the power grids, the number of micro-grids is getting more and more. But the report about the scale of micro-grid can not been seen. And so the power grid is getting more and more difficult to be analyzed, dispatched and managed, and the development and construct of the smart grid is influenced. According to the problem, based on the traditional generation cost and benefits, the cost model of micro-grid is built and the number that the power grid is divided into how many micro-grids is clear. And it is contributing to the management of power gird and the micro-grids, and then can supply a basis for simplifying power grid dispatch. Taking the IEEE118 buses system as an example, the experimental results verify the effectiveness of the proposed method.