Abstract: To deal with simultaneous order release sequence and lot sizing for manufacturing process execution, an integrated decision-making method based on simulation optimization is presented,that is composed of two basic modules: genetic algorithm module and discrete event simulation module. Segment chromosome in genetic algorithm is used to describe order release sequence and lot sizing. The order release plan evaluated by simulation model is produced through genetic operation. Order mean flowtime is taken as the objective function and its value is obtained by simulation models. The optimal results can be obtained by repeated iteration between genetic algorithm and the simulation model. Finally, the validity and feasibility of the approach has been proved on being applied in a workshop within a research institution.