Optimization of the power system plays a major role in power system planning. Unit Commitment and Economic Dispatching are the most important part of every plant-planning project. Unit Commitment is the problem that searches the economical way for power generation, when the power consumption and altered constraints of the power plants are considered. Recently, the attention of the environmental pollution problem is increasing around the world. The Emission Constrained Unit commitment (ECUC) problem is a very important issue to minimize the production of electrical power side by side with reducing the emissions of power plants while meeting different system constraints. This paper formulates a multi objective Unit Commitment problem with valve point effect considered in both fuel cost and emission function using the Particle Swarm Optimization (PSO) technique. A new approach is proposed to simplify valve point loading effect. The proposed algorithm is applied to a 10 generating units test system in one-day scheduling period at different types of price penalty factor (PPF). The power balance, generation limit, and UC constraints (such as minimum up/down time, spinning reserve, and initial state for each unit) are included in the problems formulation. The simulation is implemented in Matlab environment. The results show that there is an increase in the total operational cost when taking valve point loading effect into account.