Generally, processing a new order in a job-shop production system affects the schedule of other products. This change of schedule of other products results in some costs such as set-up cost, inventory cost, re-schedule cost. The amount of these costs depends upon the schedule (process starting time) of the new order for processing in a predetermined schedule of multiple products in a job-shop. The application of queuing theory for determining the optimal scheduling of a new order in a job-shop manufacturing system is discussed. The basic criterion considered for the new order scheduling/process starting date is the minimization of total variable cost. The total variable cost consists of the cost elements: (i) raw material waiting cost or unavoidable waiting time for the new order; (ii) inventory carrying cost for the new order due to early completion or penalty cosl due to late completion or delivery; (iii) re-schedule cost due to change in schedule of the other items. An example is given to illustrate the application of the model.