A unicycle includes an n-shaped fork and a wheel rotatably mounted between two lower ends of the fork via a drive axle. The drive axle has two opposite ends each having a crank mounted thereon. A pedal mounted on a free end of each crank. A seat post is longitudinally and securely mounted on an upper end of the fork. A seat is attached to a free end of the seat post. A curved sleeve is connected to the seat post and concentrically corresponds with the wheel. A connecting post is telescopically received in the curved sleeve. The connecting post has a free end with two opposite sides each having an L-shaped linkage mounted thereon. Two auxiliary wheels each is rotatably mounted on a free end of each linkage.