1. Abstract Recently, 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. 2. Introduction The automatic transmissions are widely used to transfer the engine power and control the vehicle speed in automobiles and other vehicles such as construction machinery. The demands for better shift quality, reliability and efficiency are getting higher and higher each year. Considering these demands, a wet friction clutch used there plays a key role to realize them. Paper type friction materials are used as the lining of wet clutches, which face and slide on the mating steel plates. Because of its porous property, the paper type wet friction material has unique lubrication characteristics which are quite different from metal. The mechanism of its friction is complicated and unresolved since it depends on various factors such as elasticity or porosity of friction material, time and temperature dependency of its hydrodynamic friction interface behavior and physical and chemical adsorptions of lubricant additives onto friction surfaces. Only recently researches on lubrication model for wet clutches have been advanced, and the mechanism is being clarified gradually 1),2),3),4). The dynamic frictional characteristics are among the most important properties of a wet friction clutch. They affect directly on torque transmission flow and the resultant shift quality. To the present in this field, Miyoshi 5) studied the relationship between porosity and friction characteristics, and Miura 6) investigated the effect of liquid permeability of friction material. Miyazaki 7) attempted to use “Strivec curve” over the practical wet clutch operation range for analyzing wet friction phenomena, and he classified it into boundary and hydrodynamic lubrication. Eguchi clarified some interrelationships among friction material, additives and viscosity of lubricant on friction characteristics by using Strivec curves. 8). Recently, AT's have been developed to have more step numbers to improve their power transmitting efficiency for better fuel economy. Because of this increased steps, the frequency of engaging and disengaging of wet clutches in an AT is also increased. Now due to the clutch-to-clutch type shift strategy, a clutch is required to react smoothly in the process of disengagement as well as in an engagement. In the past there have been many researches on the engaging process, however, few have been done for the clutch behavior in the disengagement process.