This study tries to codify a proper mathematical model which can be predict the costs of repair and maintaining the tractors as precisely as possible for three common models of tractors in Shahid Beheshti cultivation firm in Andimeshk-Ahvaz road of Dezfoul. Studied tractors include: Newhland TM155 Tractor which is chosen from the most perfect file of 10 tractor machines and Jandier tractor-3140 of 35 machines. Then we calculated the cumulative values of the two sets of data was used for different years of tractors working fitted to the final performance of the mathematical model of the six (linear exponential reverse multiple, second and third degree ) the by method of least squares and use of an computer software. With this operation, the most proper model of estimating maintenance cost for all three types of tractors is obtained as a multiplicative model and the corresponding parameters were calculated. In addition, the final model which is the best fit for a cumulative repair costs for operating hours of all tractors, was determined by multiplying. In order to determine the economic life (replacement age) 61 tractors in three different models based on price, the amount of interest rate and annual depreciation was calculated for each model and used along with the annual cost of maintenance for determining the economic life.