Abstract: Using the Motecarlo Simulation Technique, a simulation model, considering integrately the crop-mix, production technology, the operation efficiency and cost of machinery sets, suitable field days, the randomness of climate condition and its affection on available days for field operating, and relevant timeliness losses, was constructed to determine the optimum service area of varient machinery sets. Seven machinery sets for "The Region of Tai-Lake" were formed according to the principles of completeness, mutual suitability and indivisibility. On the basis of simulate-calculating and responsetesting,the economic size, and the average investment, production cost, land productivity and net income on unit area for each machinery set were fully analyzed and compared. Finally, our opinions on the problems such as the development strategy of agricultural mechanization, policies and measures, and the management system of agriculture were given.