Abstract: Target selection in calculating optimum replacement period of tractors is firstly considered according to the ownership and the use of tractors. Two calculation methods are then given respectively for different conditions. The first method is suitable for either of the following two conditions: 1) tractors are used by the owners themselves;2)tractors used for custom work with the owner and the hirer belonging to the same organization. The second method is suitable for tractors used for custom work and decision is made from the view point of optimizing the economical benefit of the tractor owners.