Faced the practical requirement of electricity markets currently implemented around the world， joint bidding in option and spot markets is addressed. A two-period Cournot equilibrium model is proposed taking transmission constraints into account. This model can be reformulated as a mathematical program with equilibrium constraints and be solved by an advanced Levenberg-Marquardt algorithm based on the nonlinear complementary approach. The results show that transmission congestion impairs producers' enthusiasm for option trading， weakens the impacts of options on electricity markets and exacerbates the market power abuse of producers further situated the constraints. When transmission network congested， producers can partly or fully expropriate revenues of consumers and network owners by strategic bidding.