The treadmill comprises a base frame adapted to rest on a surface which has left and right inner sides and left and right outer sides; a tread base having a frame including a front end, a rear end, and left and right outer surfaces; a linkage assembly having a first end connected to the front of the tread base and a second end connected to the base frame. The lateral width between the left and right outer sides of the base frame is substantially same with the lateral width between the left and right outer surfaces of the tread base; the linkage unit is approximately mounted in a space which is formed by the left and right inner surfaces of the base frame and the bottom of the tread base.