The call center considers an interesting area of the application of queueing models. In a call center's queueing model, the customers are the callers and the servers are the call agents. The call center, which effectively running must get to the balance between service level and service costs. The quality of the service presented is of essential importance. So, the call center must guarantee at any time an appropriate number of servers with appropriate multi-skills according to the expected level of the demand, this issue is ordinarily called the staffing problem. Thus , the paper has the aim to explain how to apply the queueing model method for evaluating the performance of multi-skill call center's index and computing the formula of service level through presenting the optimization of the staffing problem for the optimal number of agents in each group. Finally, we used the deduced results through a numerical example to calculate the steady- state probabilities, service levels, the optimal number of agents in each group, performance measures, and how these influence factors in the whole system.