Planetary gear sets, used commonly in wind turbine gearboxes, should have good load distribution on gears to improve the durability. In this paper, the influences of external loads caused by wind fluctuation on the load distribution on the gear tooth flank are investigated. A state-of-the-art whole system model is developed to analyze the planetary gear set for the WTG. The results of gear mesh misalignment, contact pattern, load distributions are predicted and the gear safety factor is also evaluated. Therefore, the results presented state that if the optimal helix modification is applied, the edge loading of gear tooth ends disappears, contact pattern improves significantly, the face load factor decreases and the gear safety factor increases.