Abstract: An optimum choice of mechanization process item and machimery type in development of agricultural mechanization is realized with the change of productive pattern and dynamic course of optimum factor setting. Making up a big non-linear and mixed positive number plan model is to adapt the non-linear character, which is an output out of farming input, and to show the principle of un-divisibility concerning with the technical unit of labour resource-productive tool. The productive technical structure was observated with dynamic and calculated with optimum through the productions of rice and wheat in the regions of southern Jiangsu Province, and wheat or maize in the plains of northern China, under the change conditions of labor opportunity cost and management size-service size. It revealed specificaly that the change of productive technical type and economic mechanism of mechanization development is correspondence with the relations of relative cost among factors,, It was also conditioned forcefully by the managed service size, which is related to the development of social economy, the choice of managing subject, and the growth of economic organization. Meanwhile, it proved that "the long-run average costs" is a curve of "L"type, but one of"U" type.