Over the past 170 years clothing industries have changed from custom fitting and assembly of hand sewn garment to mass production and distribution of ready to wear products in the modern market. And most of the Ethiopian clothing industries in face a problem to competitive globally because of different problems when they produce their product. For example, most of the company doesn't optimize their resources, inadequate of raw materials, poor quality. Therefore, the optimal assignment of resource for the task and proper assignment of work stations is becoming the industries competitive. This research is addressed an optimization of assembly line of long pant product through discrete event simulation (DES) modeling. In this research, the simulation modeling and optimal line balancing of the long pant sewing line is carried out by using arena 14 version. Based on the simulation model run, results the operations that have high queue length and high resource utilization are facing serging, pocket positioning, size by size zip tack to body and front fly topstitch and a resources which have been low utilizations or below 50% utilizations are like BH machine, manual operator2, helper a2, and heat machine are idle operations in the production lines of the long pant model. And the operations that have high waiting time are bottlenecks for the production lines. identified bottlenecks are eliminated or minimized using sharing and adding of the existing resources (operators and machines) in order to optimize the resources as much as possible.Finally, a new model has developed by using opt quest model of arena with different types of scenarios by considering the constraints of the resources and controls and responses of the results. Therefore, the production system produces 498 long pants and by reducing one operator and one 4TOL machine. The researchers have identified the main bottle necks which have high work in process and low work in process, high queue length, low and high utilized resource specially the operators and the machines that allocated in the assembly lines.