The inter-connection between generation investment and transmission investment is usually neglected in traditional methods of electric power planning. Unable to deal with the uncertainties of investment value effectively and the influences from other power companies'（Powcos） investment strategies， these methods could hardly meet the requirement of power market. Based on the Black-Scholes option pricing theory and non-cooperative game theory， a deferment option game model of generation investment and transmission investment is developed， which makes the strategies of Powcos' investment more rational and the electric power network planning more harmonious. The rationality and validity of the method proposed is illuminated by a practical test example.