This paper proposes the use of optimal power flow (OPF) to solve a multi-objective optimization problem (MOP) of environmental/economic dispatch (EED). Firstly, by using an aggregate objective function (AOF) with a set of uniform weight vectors, the EED problem is decomposed into a group of scalar sub-problems, each of them is aggregated into a single objective. The uniform weight vectors represent different trade-off levels between each objective (i.e. fuel cost and emission). Secondly, each sub-problem is optimized by the similar numerical analysis of OPF rather than heuristic algorithms, while taking into account the losses in the transmission lines, power balance and the generating unit active and reactive power constraints precisely. Thus, an accurate optimum can be obtained for each sub-problem rather than a reasonable optimal solution every time. Further, each optimum is then associated with a concrete weight vector which can be treated as decision maker (DM)'s preference for selecting the final preferred solution. The simulation results based on the IEEE 30-bus system have shown that the obtained solutions are accurate and stable, thus they can be used as benchmark to test other methods as well.