1Recently, automatic transmissions (AT) have been developed to have more step numbers to improve their power transmitting efficiency for better fuel consumption. Due to the increased steps, the frequency of engaging and disengaging of wet clutches and brakes in an AT is getting higher. And then the required characteristics for a clutch is not only to make a smooth engagement but also to react smoothly in the process of disengagement, due to the clutch-to-clutch type shift strategy. Although there have been many researches on the engaging process, however, few have been done for the clutch behavior in the disengagement process so far. Regarding the process of disengagement of a clutch, a phenomenon described below has been observed and reported which leads to a kind of shock during a shift.In the process of one clutch disengagement, the clutch starts to be opened with some slipping at very low apply pressure from the cramped state and, during the slipping, sometimes a friction rise in the clutch is observed and recognized as a shift shock by the driver. We would like to call it as “mu-swing.”The purpose of this study is to eliminate or reduce mu-swing under this very low facing pressure in a clutch. In order to investigate factors which influence mu-swing, a series of evaluation with varied conditions has been done, and the important influence of the state of oil film at sliding surfaces was confirmed. The factors which prevent mu-swing were also evaluated, with respect to the friction material type and oil groove patterns.