This paper proposes design decision making methods of machine structures based on design sensitivity analyses for minimizing the influence of variations in the manufacturing processes on the structural characteristics (product performance). Design decisions are made so that smaller design sensitivity coefficients of the characteristics are given for areas that have been estimated to have a great potential for variation in the design variables. First, fundamental design principles are established for minimizing variations of the characteristics. Then, the practical procedures are presented for decreasing the variations of the product performance. The proposed method in which the minimization of variations of the product performance is the main objective corresponds to a maximization of the product performance and also brings about a decrease of the manufacturing cost. Finally, the effectiveness of the design decision making methods are demonstrated for designing a machine tool structure.