摘要：The goal of the modern farming systems is to economize energy consumption and to reduce farming *** selection and use of agricultural machines are important factors to achieve this *** tillers are tillage tools that are used for accomplishment of both the primary and secondary tillage *** the widespread application of rotary tillers and wide use of modern tractors in Iran,optimal design of these machines is *** in this study,the optimal working width and optimal diameter of the rotary tiller's rotor proportionate with the power of the MF 399 tractor was delineated based on the energy *** theoretical method is based on the specific works of tractor and rotary *** showed that the optimal working width and optimal diameter of the rotary tiller’s rotor with maximum of forward speed 1 m/s for tractor was 240 cm and 6.27 cm,respectively.