Abstract: A yo-yo includes a left yo-yo half cap, a left half yo-yo body, a clutch mechanism, a right yo-yo half body and a string. A shaft fixed on the left yo-yo half cap fits through a shaft sleeve of the clutch mechanism and the left yo-yo half body and firmly in the right yo-yo half body for an inner end of a string to tie and wind around on to let the yo-yo move up and down with the string. The clutch mechanism has two compression springs respectively located at two sides of the shaft sleeve and fitting in a space in the left yo-yo half cap and two weighted balls respectively located between the shaft sleeve and the springs. The clutch mechanism functions to force the balls to engage and disengage the shaft sleeve or, letting the yo-yo rotate and wind/unwind itself on the string or rotate idle on the end of the string.