This paper investigates generators' strategic behaviors in contract signing in the forward market and power transaction in the electricity spot market. A stochastic equilibrium program with equilibrium constraints (SEPEC) model is proposed to characterize the interaction of generators' competition in the two markets. The model is an extension of a similar model proposed by Gans, Price and Woods  for a duopoly market to an oligopoly market. The main results of the paper concern the structure of a Nash-Cournot equilibrium in the forward-spot market: First, we develop a result on the existence and uniqueness of the equilibrium in the spot market for every demand scenario. Then, we show the monotonicity and convexity of each generator's dispatch quantity in the spot equilibrium by taking it as a function of the forward contracts. Finally, we establish some sucient conditions for the existence of an local and global Nash equilibrium in the forward-spot markets. Numerical experiments are carried out to illustrate how the proposed SEPEC model can be used to analyze interactions of the markets.