The two agricultural zones, mid-and-northen region and southern region, were established according to the possibility of double cropping in the paddy field. For each agricultural zone typically mechanized farming systems with a power tiller for cultivating rice, barley and soybean were established. A computer program, which determines the optimum size of power tiller for the given farm size, the ratio of paddy area to total area and the ratio of double cropping area to the paddy area, was developed. The computer program was executed for the farm sizes, 1 to 3 ha, and the ratios of paddy area to the total area, 60~80%, and the ratio of double cropping area to the total paddy area, 70%. The following conclusions were drawn from analyzing the computer outputs. 1. In the mid-and-northern region, a smaller power tiller (5 or 6 PS) appeared to be more economical for the farm size up to 3 ha. From the viewpoint of fuel consumption a 6 PS power tiller appeared to be more favorable. 2. In the southern region, a smaller tiller (5 or 6 PS) appeared to be more economical for the farm size up to 1.5 ha. But, a larger power tiller (8 or 10 PS) appeared to become more economical as the farm size and the barley planted area increased. 3. For the southern region where rotary tilling is heavily required due to double cropping, it is recommended to develop a power tiller which weighs light like the conventional 5 or 6 PS small power tiller but has lager power.