In this paper, the productivity estimation of the painting line in an automobile plant is studied. To improve the bottleneck and the weak point of the process, computer simulations are performed. In determining the system specification, the status of the conveyor lines and the other physical facilities are considered in order not to intervene with each other and to satisfy the painting facility restrictions. As simulation parameters, production performance ratio, number of hanger and dolley, down time, speed of conveyor and the pitch of a painting body are chosen on the basis of exhaustive field evaluation to study their effects on the capacity of the process. The results of the simulation show that we can improve the capability of they system without additional investment to the plant facility. The best condition for the maximum capacity of the process is selected by comparing the alternative computer simulation results. As a simulation language, Promodel is used which is very useful and easy to use in manufacturing oriented simulation.