Abstract The frictional performance of the wet clutch used for automatic transmission is determined by the interaction between the paper- based friction material (D/C), separator plates (S/P), and automatic transmission fluid (ATF). In order to understand the impact of additives in ATF on the frictional performance, commercial ATF was modified by adding 5 types of additives one by one in a specified concentration, and the friction test was conducted for 6 levels of A TF consisting of these 5 modified ATFs plus commercial ATF. The friction test consisted of a dynamic test conducted for 2000 cycles interspersed with a static test conducted once every 100 cycles, in order to measure the coefficient of static friction (μs).As a result, it was found that only in the friction test conducted by using ATF with the addition of sulfur-based friction modifiers, there was an increase of 30% from the existing coefficient of static friction (μs) up to 1500 cycles, but at 2000 cycles, it dropped to the same value as the μs using ATF with no additives. Then, surface analysis was conducted for the frictional surface when the μs was high, as well as when the μs dropped, by X-ray Photoelectron Spectroscopy (XPS) and Time-of-Flight Secondary Mass Spectrometry (TOF-SIMS), in order to clearly identify the relationship between deposits on the frictional surface and μs. Introduction While automobiles are becoming more and more convenient and comfortable, this must necessarily be balanced against the need to make them lighter, as a result of which high μs is required from the wet clutch, which has been made smaller, and the number of plates in the clutch has been reduced in order to make the automatic transmission smaller and lighter. The frictional performance of a wet clutch depends on the interaction between the paper-based friction material, separator plates (S/P), and the automatic transmission fluid (ATF), and several effects of ATF additives on the coefficient of static friction have been reported 1). In order to apply this to the development of future friction material, the relationship between the frictional performance of existing friction material and ATF additives was understood, and by identifying the deposits on the frictional surface after the friction test, their impact on the frictional performance was clearly shown. 1. Friction Test The friction test was conducted by using a total of 6 levels of oils, namely, commercial ATF(ATF-A), and commercial ATF which has been modified by adding Calcium-based detergents (Ca_Det), Polymethacrylate viscosity modifiers (PMA_VII), Molybdenum-based friction modifiers (Mo_FM), Phosphoric acid-based friction modifiers (P_FM), and Sulfur-based friction modifiers (S_FM) respectively in a specified concentration. Paper-based friction material from our company was used as wet friction material (D/C), and low carbon steel was used as the separator plates (S/P) which functioned as the accompanying material. The friction test consisted of a dynamic test conducted for 2000 cycles using an SAE No. 2 testing machine, interspersed with a static test conducted once every 100 cycles, in order to measure the coefficient of static friction (μs). The friction test conditions are shown in Table 1. Table 1. SAE No. 2 conditions (JASO M348) Fig. 1 shows the summary of the variations in μs between the 6 levels of the friction test. The tests using ATF to which PMA_VII and Mo_ FM were added, showed the same variations in μs as the test using ATF-A, and the test using ATF to which Ca_Det was added showed low μs by 0.04 from the first cycle up to 2000 cycles. The test using ATF to which P_FM was added showed a high μs by 0.01 which remained stable up to 2000 cycles, while the test using ATF to which S_FM was added, showed an extremely high μs up to 1000 cycles (30% higher than the existing value), but at 2000 cycles, it dropped to the same value as the test using ATF-A.Impact of Organic