The entrance rate of vehicle bodies into assembly lines to complete the assembly process is so important for managing and increasing the daily production. In this paper, using discrete simulation technique and a simulation methodology, which is designed by simulation technical committee of Michigan University, we modeled and simulated the mechanical assembly line of RIO vehicle in SAIPA Corporation to find the optimum entrance rate, such that the daily production and total productivity are increased, besides the entrance rate and cycle time are shortened, reasonable and feasible as simultaneously. The entrance rate of RIO vehicle bodies into mechanical assembly line is one vehicle per 6 minutes. The daily output is in arrival [53, 69]. After various running of simulated system of mechanical assembly line of RIO, we found its optimum status such that the outputs and productivity are increased. In this regard, if the entrance rate is reduced as much as 2.2 minutes, the daily output of mechanical assembly line will be in arrival [65, 77], where, the lower bound of the assembly line production will increase as much as 12 units and the upper bound of daily output rate will increase as much as 8 units or vehicle bodies.