This paper has proposed the modelling by simulation in order to evaluate the performance of process through discrete event simulation using SIMAN language in flexible manufacturing systems. The production system is assumed to be a job shop type of system under the batch production of discrete products. In this paper, the input data is the workstation(process) time, the number of workstation(process), a probability distribution, the number of simulation runs. Also, transient period is considered. In the case study, this paper deals with three products in real flexible manufacturing systems. Finally, a number of simulation runs were executed under different experimental conditions to obtain preliminary statistics on the following performance measures: operating rate of facility and average system operating rate, transient period, central processing unit, average system throughput, and average waiting time in queues.