This paper is concerned with the motion of water-droplet on rotating disk.It is found that the contact angle hysteresis [1] plays the essential role to deviate the droplet against the Coriolis force due to the rotation of the disk.Experimental: Figure 1 depicts a sketch of the present experimental apparatus; rotation speed is of 237rpm,and the rotation direction is clockwise(CW).A droplet of 10x10-2(cm3) is placed on the smooth compact disk of 120mm in diameter and 1mm in thickness.The room temperature is of 23℃.Rotation speed of the disk is set to be of 237rpm,and the rotation direction is either clockwise (CW) or counter clockwise △(CCW).The disk is accelerated with 10 rpm/s,and reaches at the final speed of 237rpm,which speed is kept for 300 seconds.Photographs are taken by CCD-camera at the shutter speed of 40x10-6 second.Stroboscope spark frequency is of 441.67(s-1),so the spark duration is of 2.26x10-3 second.The stroboscope spark is synchronized with the camera shutter.Results and Discussion: Figure 2 depicts the loci of the water droplet on the rotating disk,while Figure 3 illustrates the one shot.The rotation speed is of 237rpm,and the rotation direction is clockwise (CW).Point A:t=0-34,B:t=35-48,C:t=49,D:t=50,E:t=51,△t =2.26x10-3second,where t is the sequential number of the camera shot,and △t the time interval between the adjacent two shots.Contrary to the expectation,it must be noted that the droplet is deviated to the right facing to the direction of motion,following the curved loci from t=0 to 51,and then goes to the left by crossing the center line and disappears at t=52.This does mean that the surface tension towards the right exceeds the Coriolis force towards the left facing to the direction of the droplet motion during the period from t=0 to 51,even though the latter force overwhelms the former after t=52.Concluding Remark: Contrary to Thomas Youngs proposition [2],the contact angle of water on a solid surface is not unique.It is found that the contact angle hysteresis between the water and the disk is the primary factor to deviate the droplet to the transverse direction facing to the direction of its motion.