Abstract: The elastohydrodynamic lubrication (EHL) of filtering reducer was studied by considering the tooth correction, the real tooth surface roughness and transient effect. A mixed lubrication model was established. The maximum pressure and central film thickness were calculated at different meshing points with different longitudinal modification parameters. The effects of roughness surface and speed on lubrication performance were obtained. The results show that both of modification parameters, r and R_y, can be optimumized to make the maximum oil film pressure decrease significantly and hence avoid the edge effect in meshing process. The increase in parameters, r and R_y, increased central film thickness. For the gear tooth without longitudinal modification, the maximum oil film pressure increased significantly because of the roughness surface effect, but decreased after longitudinal modification. Therefore, longitudinal modification is quite important for gear reducer as the contact surface was rough. The decrease in speed reduced average film thickness but increased contact area ratio, which makes the lubrication from full-film and mixed EHL to boundary lubrication.