Background: Agricultural tractor is the agricultural machinery which conducts several operations using various implements such as harvester, baler and trailer. Power from the engine is directly transmitted to driving shafts and PTO (Power Take Off) shaft. Power delivered to PTO shaft is transmitted to the implements through the PTO gear. The gears are components of transmission which transmit the power of an engine to a machine and offers various speed ratios, a compact structure and high efficiency of power transmission. A gear train design in the automotive industry uses simulation software. However, the PTO gear design for agricultural applications uses the empirical method because of the wide range of load fluctuations in agricultural fields. Methods: Therefore, severity analysis with agricultural operation of PTO gear using a simulation is essential to the optimal design of the PTO gear. In this study, severity analysis using a simulation proceeded in full load condition and real work condition (70% rotary tillage, 30% baler operation) during the rotary tillage, baler operation. Results: Simulation results showed the severity of full load condition was the higher than the real work condition. Discussion: This study secured the reliability of the PTO gear design through the severity analysis, and it also proved the feasibility of implementation of the optimal design by using gear analysis software. Conclusion: Thus, it is necessary to design the tractor PTO gear in consideration of the field data.