Agricultural tractors are designed with empirical method because it is not easy to measure precise load cycle under various working conditions and soil types.Especially, PTO driving gears directly drive various tractor implements.Therefore, alternative design methods using a gear design program are needed for optimal design of tractors.The PTO driving gears are made with SCr415 alloy steel with the treatment of carburizing and quenching.The full load on PTO driving gears was used, and the load was measured during actual field works of plow and rotary tillage.Stress, safety factor, and fatigue life were simulated and analyzed under various load conditions.The simulation results showed that PTO driving gears were safely designed because safety factor was greater than 1.5 under various load conditions.Stress of PTO driving gears with full load condition was higher than one from the field load condition.In contrast, safety factor and fatigue life under full load condition were lower than the one from field load condition.This means that full load condition was severer than the field load condition to the tractor PTO.Therefore,designing PTO driving gears should consider the field load rather than full load.