A folding treadmill includes a tread base supported by a foot structure which has guide plates formed with elongated slots on two sides of the tread base. The tread base is connected pivotally on the base support through pivot members which are mounted movably within the respective slots so that the tread base is turnable between a folded position and an unfolded position and movable rearward when the tread base turns to the folded position or forward when the tread base turns to the unfolded position. The foot structure further includes a guide rail structure on two sides of the tread base to support movably a first roller which is attached to the tread base and a lifting device for lifting the tread base.