In this paper, demand response modeled with unit commitment in a nonlinear optimization problem, for increasing system security and economic reasons. By incorporating the demand response in the market it should decrease cost reduction, market clearing problems, congested lines and make a flat load profile at peak hours. In this problem, loads are combined of fixed and responsive loads. When this loads participate in program, they could curtail or shift to another operating hours. By considering system security, Studies show system speed and higher accuracy in problem convergence. Also, demand response decrease network load peak, system operating cost, fuel consumption and line congestion. Numerical simulation shows the effectiveness of proposed method on 6- bus system