The anti-shudder effect of ATF additives and their mechanisms have been investigated. Anti-shudder durability was evaluated using an automatic transmission (AT) on an engine stand under continuously slip-controlled condition. The addition of over-based Ca-sulfonate and friction modifier (FM) remarkably improved the anti-shudder durability of ATF.The surface roughness of the contact area (contact area roughness) of the clutch plates was measured by an electron probe surface roughness analyzer. To evaluate the boundary frictional properties of the adsorbed film formed, the friction coefficient of the clutch plates in the absence of oil was examined after the anti-shudder durability test.It was found that shudder occurrence was strongly correlated with the contact area roughness and the boundary frictional property of the steel plate surface. Large contact area roughness and low boundary friction were preferred to prevent shudder. It was suggested that the increase in the contact area roughness was caused by over-based Ca-sulfonate, and that the decrease in the boundary friction was caused by over-based Ca-sulfonate and FMs.