A computer model was developed to aid farm machinery decision makers in deciding the optimum replacement time for an individual machine. The model based on solving a set of mathematical equations via Microsoft Visual Basic® to resolve the appropriate decision. The mainly input data composed of machines purchased price, date and age when purchased, the annual inflation and interest rates, the yearly repair and maintenance costs and operating hours over the machine's life. The major criterion to keep equipment in service or replace it was the values of calculated average accumulated costs over a period of years. To run the model, realistic costs data of Kubota combine (35.79 kW), collected from the Agricultural Engineering Station in Elsadeen – Sharkia governorate, were utilized to proof its capability of making decisions. The results showed that it might be better to replace the Kubota combine at the end of year eighth years old or after 6000 operating hours. With high confidence one can assume that the current model would be helpful in assisting the mangers of farm machinery in building a clear strategy for machinery replacement.