Abstract: The helical gears were discretized into many small segments along the contact line and each segment was viewed as a spur gear with an equivalent angular velocity.The radii of curvatures and surface velocities in the direction that was normal to the contact line were obtained through Euler's equation. The mixed thermal EHL model of helical gear was built based on load sharing concept, elastohydrodynamic lubrication theory and rough line contact theory. The performance of helical gears with rough surfaces with considering thermal effect was predicted using Newtonian model and Carreau rheological model, respectively. The results show that variations of the film thickness, portion of load carried by film and film temperature rise with the meshing time and the contact line calculated by the two rheological models were identical. The friction coefficient calculated by Newtonian model was beyond the project reality. The friction coefficient calculated by Carreau rheological model matched with the project reality and its variation with meshing position along the contact line was contrary to the variation of film thickness.