This　paper　aims　to　identify　the　effects　of　tooth　modification　amount　on　the　dynamics　of　helical　gear　system.　For　this　purpose,　a　driven　helical　gear　pair　in　a　hydro-mechanical　transmission　is　taken　as　a　study　case　for　which　an　Eight　Degree　of　Freedoms　(8-DOFs)　dynamic　model　is　established　by　considering　the　bending-rotation-torsion　effects.　Besides　the　component　deflections,　the　profile　deviations　from　tooth　modification　are　also　included　and　further　integrated　into　the　dynamic　model　to　investigate　their　effects　on　gear　dynamics.　By　taking　the　dynamic　meshing　force　and　dynamic　transmission　error　as　indices,　the　influences　of　tooth　modification　amounts　on　the　dynamic　behaviors　of　the　transmission　system　are　explored.　The　results　show　that　an　appropriate　gear　modification　can　effectively　suppress　the　dynamic　responses　of　helical　gear　system.　And　the　proposed　dynamic　model　provided　theoretical　fundamentals　for　vibration　suppression,　noise　reduction　and　durability　enhancement　of　the　transmission　system　in　the　subsequent　design　stage.　©　2017　Taylor　&　Francis　Group,　London.