A gearbox is a power transmission system consisting of various machine elements such as gears, bearings, shafts, and housings. The gearbox has the advantage of being able to change the torque and rotation speed according to the gear ratio and has high power transmission efficiency by transmitting power through the contact of the gear pair. Although improvement and optimization of gearbox performance was researched in various fields, it is being conducted using an equivalent load condition rather than an actual workload. If the gearbox is evaluated using the equivalent load condition, the evaluation results are less reliable. Therefore, the gearbox must be evaluated under the actual workload condition in which the gearbox is operated. In this study, the evaluation of the gearbox was performed based on the actual workload measured in the environment in which the gearbox operates, and design modifications were made to improve and optimize the gearbox performance. As a result, shaft deflection was reduced by rearrangement of the bearing, from an overhung type to a straddle type, thereby improving the fatigue life of the bearing. Finally, the load distribution acting on the gear tooth surface was improved through micro-geometry modification of the gear.