Since agricultural machinery performs work on various environments and crops, the design and development of agricultural machinery should be performed in consideration of the actual working load. Therefore, this study was performed to measure the actual working load of the maize harvester. In addition, a load spectrum was built to evaluate the strength and life of gearbox elements in the maize harvester using the measured actual working load. The load spectrum was built by dividing it into load duration distribution, which is applicable to gears and bearings, and load spectrum, which is applicable to other components except for gears and bearings. The load duration distribution & load spectrum built through this study is expected to be used not only for rating the strength and life of the gearbox of the maize harvester but also for the design load of the gearbox.