The research work presented in this paper deals with resources allocation in a production system in order to meet a given production specification. More precisely, we focus on human resources optimisation in design phase of a manufacturing system. We consider that operators may have different qualifications. A highly qualified operator is more expensive, but it is more efficient too. Firstly, we propose a linear programming based approach to calculate the appropriate number of operators and the requested level of skills in order to minimize labor costs for a given productivity. The result is used to generate a scheduling which minimizes the work in process.