Abstract: An accurate calculating approach for flash temperature of internal helicalgears is presented. Firstly, the load distribution on tooth surface is obtained by using 3-D finite element method and loaded tooth contact analysis technique. Then, the relative velocities and curvature radii of every node on the tooth surfaces for a pair of internal helical gears are determined. Under these conditions, the flash temperature on tooth is calculated by using Block's basic equation. Further, the thermal elastohydrodynamic analysis of line contact is applied to determination of the temperature distribution in oil film. This analysis is more accurate, though more tedious. The calculated results reveal the relationship between the gear parameters and flash temperature, which will help engineers in designing internal helical gears with high scuffing load capacity.