The number of years of depreciation (NYD) is counted from the first use of a new machine through its disuse. The NYD of a kind of forest machinery is determined by its aveiage annual cost during the use in a forest legion year after year, and the data are computer-optimized by means of comparing them point by point and machines one by one, analyzed statistically and finally examined by experts. The optimum NYD is 6~3 years for transport cars, 6~7 for logging tractors, 10 for yarders and 15 for forest locomotives. Compared with the old standard, the new one can raise working efficiency by 26~46 percent and reduce the cost by approximately 1000 yuan (RMB) per year for one car. Furthermore it will be benificial to the machine maintenance and management.