This paper presents Particle Swarm Optimization (PSO) technique to solve economic dispatch of valve-point loaded generating units considering emission constraint. This problem has gained recent attention due to the deregulation of power industry and environmental regulations. Minimizing operating cost can no longer be the only criterion for dispatching electric power due to increasing concern over the environmental consideration. In this paper, fuel cost and NOx emission functions are considered and formulated as a single objective optimization problem. Based on the literature survey, it could be found that cost function is taken as a quadratic function and solved for emission economic dispatch. Here, in cost function a sine term is added to model the valve-point effect and then solved using PSO algorithm. The objective function is highly non-linear and the proposed method is validated with IEEE 30-bus system. The results obtained demonstrate the effectiveness of the proposed method for solving the environmental constrained economic dispatch problem.