Optimized distribution on whole stiffness of machine tool can improve the performance of machine tool and cut the cost. Taking a precise numercial control (NC) lathe as an example, combining the multi-body system theory with the homogeneous coordinates transformation, A mapping relationship model between the whole stiffness of machine tool and its components is established. The change value of whole stiffness caused by component stiffness was calculated by this model. According to these calculations, the best effective components and its optimized direction were selected. Taking the minimum cost as an optimized goal, optimized distribution of major component stiffness is finished by the genetic algorithm. This method can improve the efficiency and result on stiffness control of machine tool.