An ankle exerciser comprising: a base having a bar and a frame, the bar transversely connected to the frame and including two installation positions; two pedals respectively installed to the bar, each pedal including a board and a first belt which is mounted to the board, a socket connected to an underside of each of the two pedals and including a plate, a brace and a ball bearing, the plate located between the underside of the pedal and the brace, the brace including a room in which the ball bearing is located, and two rotary units respectively located between the base and the two pedals so that the two pedals are rotatable by the two rotary units, each rotary unit including a shaft, a cup, a damp unit, a collar and a joint unit, a bolt extending through the shaft, the cup, the damp unit, the collar and the joint unit so as to be connected to the joint unit, the shaft located on a top of the bar and located corresponding to the installation position, the shaft including a passage defined axially through, a bottom portion formed to a lower end of the shaft, the cup contacting the bottom portion, the damp unit located between the cup and the collar, the collar positioning the socket and including a receiving portion facing downward in which the damp unit is received, the joint unit including a ball formed to a top end thereof, the ball bearing mounted to the ball, a lower section of the joint unit inserted into the passage of the shaft, the bolt connected to a threaded hole in the lower section of the joint unit, when in use, each of the pedals is pivoted about the joint unit, and each of the pedals returns by the damp unit. a base having a bar and a frame, the bar transversely connected to the frame and including two installation positions; two pedals respectively installed to the bar, each pedal including a board and a first belt which is mounted to the board, a socket connected to an underside of each of the two pedals and including a plate, a brace and a ball bearing, the plate located between the underside of the pedal and the brace, the brace including a room in which the ball bearing is located, and two rotary units respectively located between the base and the two pedals so that the two pedals are rotatable by the two rotary units, each rotary unit including a shaft, a cup, a damp unit, a collar and a joint unit, a bolt extending through the shaft, the cup, the damp unit, the collar and the joint unit so as to be connected to the joint unit, the shaft located on a top of the bar and located corresponding to the installation position, the shaft including a passage defined axially through, a bottom portion formed to a lower end of the shaft, the cup contacting the bottom portion, the damp unit located between the cup and the collar, the collar positioning the socket and including a receiving portion facing downward in which the damp unit is received, the joint unit including a ball formed to a top end thereof, the ball bearing mounted to the ball, a lower section of the joint unit inserted into the passage of the shaft, the bolt connected to a threaded hole in the lower section of the joint unit, when in use, each of the pedals is pivoted about the joint unit, and each of the pedals returns by the damp unit.