We focus on the method to generate the order of parts to assembly products considering the work difficulty in the parts assembly process. The quantitative evaluation value of work difficulty is created by using motion analysis ‘MTM’ for the parts assembly process and the mathematical model is constructed to generate the order of parts to assembly. In this study, we explain the characteristics of the quantitative evaluation value of the work difficulty and the mathematical model developed to generate the order to minimize the total work difficulty. Numerical experience is performed to evaluate the performance of the mathematical model, and the results obtained by the mathematical model are investigated by comparing the resultant operation time obtained from actual parts assembly work.