An exercise device comprises a frame, a rotational mechanism, a left and a right coupling mechanism, and a pace-adjusting mechanism. A supporting structure is upwardly extended from the front end of the frame. The rotational mechanism is arranged at the supporting structure. The left and right coupling mechanisms are arranged at a side of the supporting structure, respectively. One end of the left and right coupling mechanism couples to the rotational mechanism, and the other end respectively comprise a left and a right pedal assembly for making a moving path. The pace-adjusting mechanism comprises a left and a right linkage assembly, a left and a right swing assembly, and a driving device. The pace-adjusting mechanism can alter the pace length of the left and right pedal assembly.