Three-dimensional elastic stress near the tooth root of involute spur gear is analyzed in detail using finite element method in conjunction with sub-modeling technique provided in ANSYS.Emphasis is placed on the relationship between critical root stress（CRS） and the face width.Results indicate that root stresses noumiform distribution along the face width

and the maximum CRS and the corresponding position are relevant to the width.The total displacement in the tooth root is not the same along the face width and varies distinctly near the surface.