The recent development in the eco-friendly energy industry has motivated many researchers to focus on wind turbines. The advancement of wind turbine technology is in accordance with the trends in the last century. Pitch systems, which comprise wind turbines, have a high failure rate. However, the fatigue life of pitch systems has not been sufficiently studied. A pitch system, which greatly affects the power generation efficiency of a wind turbine, consists of a motor, a gearbox, and a pinion. In this work, a fatigue life analysis of a pinion is performed to improve the durability of pitch systems. Frequent changes in rotation are unique to a pitch system in operation. When an impact load exists at an inverse turn, the stress generated from the impact load is the dominant factor to consider in evaluating the fatigue life of a pinion. The stress history at the gear tooth is determined through a finite element simulation. The cumulative damage fraction is calculated via rainflow cycle counting on the basis of the Goodman relation and Miner's rule. Therefore, we propose a method for evaluating the fatigue life of a pinion, which considers the effect of an impact load. A conservative design of pitch systems is needed to ensure the reliability of wind turbines under an impact load.