Relationships between vehicle's high speed stability during a steering action and following aerodynamic coefficients have already been reported in the past: coefficients for time-averaged aerodynamic lift, yawing moment, side force and rolling moment. In terms of the relationships, however, we have occasionally experienced different high speed stability during steering input even with identical suspension property and almost the same aerodynamic coefficients. A vehicle during high speed cornering shows complex behavior due to unsteady air flow around the vehicle and unintentional steering input from a driver. So it is supposed that the behavior is too complex to be fully described only with those aerodynamic coefficients.Through on-road test  and CFD analysis [2,3,4], we have studied unsteady aerodynamic characteristics around a vehicle for pitching motion during straight-line high speed driving. As a result, the existence of aerodynamic damping to stabilize the pitching motion was confirmed. The result also showed that unsteady aerodynamic characteristics had a substantial influence on the vehicle stability during straight-line high speed driving. The unsteady characteristics are assumed to affect lateral and yawing vehicle motions during cornering as well. In recent years, several studies on unsteady aerodynamic characteristics for a yawing vehicle have been conducted in wind tunnel [5,6], but not yet enough for on-road test.In this study, we attempted to clarify how the unsteady air flow around a steered vehicle influenced on the vehicle during high speed cornering on road. In the first report of the two for this time, we examined vehicle motions and aerodynamic forces loaded on the vehicle.The results of analysis on the vehicle motion and its surface pressure fluctuation indicated that unsteady aerodynamic forces to hold back the vehicle cornering motion were created there. It was also confirmed, in a comparison of two vehicles having different high speed stability during steering input, that the more stable vehicle had greater aerodynamic forces to hold back the cornering force. Thus it was suggested that the unsteady aerodynamic forces swayed the vehicle stability during high speed cornering.