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Abstract. Agricultural tractors are generally used for variety of works at a number of countries as multipurpose agricultural machine. Gear system of transmission which took large proportion of the tractor price is most important. The forward 16 and 16 backward working speeds of a 55 kW agricultural tractor were determined by the gear system in Korea. A main gear system among the gear system decides the forward 4 and 4 backward working speeds. Therefore, the purpose of this study was to analyze effects of working speeds on the main gear system of the 55 kW agricultural tractor using a gear simulation software. The input load data into driving gear is rated torque and rotational by the working speeds. Then, safety factors and fatigue life of the gears were calculated based on ISO 6336 and Modified Miner‘s rule using the gear simulation software, respectively. As a result, when the working speeds of the agricultural tractor increase, the safety factors and fatigue life of the gears were decrease. Therefore, farmers should determine working speeds of the agricultural tractor by considering the load torque and durability of transmission.