摘要: The goal of the modern farming systems is to economize energy consumption and to reduce farming costs.Proper selection and use of agricultural machines are important factors to achieve this end. Rotary tillers are tillage tools that are used for accomplishment of both the primary and secondary tillage practices. Considering the widespread application of rotary tillers and wide use of modern tractors in Iran, optimal design of these machines is mandatory. Therefore 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 method. This theoretical method is based on the specific works of tractor and rotary tiller. Results 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. Keywords: Rotary tiller, optimal design, working width, rotor, MF 399 DOI: 10.3965/j.issn.1934-6344.2009.02.001-007