This paper presents a new Three-Stage (THS) approach for solving Unit Commitment (UC) problem. The proposed method has a simple procedure to get at favorite solutions in a feasible duration of time by producing a primal schedule of status of units at the first step. In the second step the operating units take hourly values by doing Economic Dispatch (ED) on them via a hybrid serial algorithm of Artificial Intelligence (AI) including Particle Swarm Optimization (PSO) and Nelder-Mead (NM) algorithms. In spite of the acceptable solutions obtained by these two stages, the presented method takes another step called the solution modification process (SMP) to reach a more suitable solution. The simulation results over some standard cases of UC problem confirm that this method produces robust solutions and generally gets appropriate near-optimal solutions. (C) 2010 Elsevier Ltd. All rights reserved.