It is pointed out that， in the situation of power plants separated from the grid， generation side having competition and regional electricity markets having not been founded， it's exigent for dispatching organizations， generation corporations， the government and electricity regulatory departments to construct evaluation indexes for impartial and open dispatching which includes the impartiality of implementing annual generation plans. After analyzing the course of making and implementing annual generation plan， the electricity accomplishment rate of generation units is defined， and the rate is then modified by taking electricity into account in which generation units can't or are deprived to generate. Referring to Lorenz curve and Gini coefficient in economics， the impartiality evaluation index for annual generation plan implementation is constructed and its fundamental calculation method is illustrated. On the basis of generation data in 2006 of power plants directly dispatched by North China Power Grid， three cases are given in order to illustrate that the impartiality evaluation index can properly respond the decentralization degree of generation plan accomplishment rate， and indicate the impartiality degree when the power dispatching organizations do their jobs.