As　one　of　the　most　influential　factors　leading　to　gear　vibrations,　transmission　errors　of　the　engaging　gears　must　be　controlled　to　achieve　a　desirable　dynamic　performance　for　a　power　transmission　system.　It　is　well　known　that　tooth　modification　is　an　effective　way　to　reduce　the　fluctuations　of　the　transmission　error　of　a　gear　pair.　The　challenge　is　determining　how　to　establish　a　quantitative　relationship　between　the　tooth　modification　parameters　and　the　transmission　error　fluctuations　of　a　gear　pair.　The　present　study　aims　to　reveal　the　sensitivity　of　the　tooth　modification　parameters　on　the　transmission　error　fluctuations　of　a　helical　planetary　gear　train　in　a　wind　turbine　gearbox.　For　this　purpose,　a　sophisticated　parametric　three-dimensional　contact　model　that　included　the　micro-geometries　of　the　tooth　modification　is　developed　in　the　ROMAX　(R)　environment.　Based　on　this　model,　a　loaded　tooth　contact　analysis　is　carried　out　to　compute　the　meshing　characteristics,　such　as　the　contact　pressure　and　transmission　error　of　each　gear　pair　in　the　planetary　gear　train.　With　the　obtained　meshing　characteristics,　the　tooth　modification　amounts　of　the　engaging　gears　were　determined　using　empirical　formulas.　These　modification　amounts　are　designated　as　the　mean　values　of　the　samples　generated　by　the　central　composite　method.　After　repeating　the　loaded　tooth　contact　analysis　process　for　each　generated　sample,　a　quadratic　polynomial　function　is　derived　using　the　response　surface　method　to　describe　the　quantitative　relationship　between　the　tooth　modification　parameters　and　the　dynamic　transmission　error　fluctuations.　A　large　number　of　random　samples　are　generated　using　a　Monte　Carlo　method,　and　the　corresponding　dynamic　transmission　error　fluctuations　are　determined　with　the　aforementioned　quadratic　polynomial　function.　Based　on　these　samples,　a　reliability　sensitivity　analysis　is　carried　out　to　demonstrate　the　effects　of　the　tooth　modification　parameters　on　the　dynamic　transmission　error　fluctuations　of　the　helical　planetary　gear　train.