A folding treadmill includes a base frame provided with a stop member, a treadbase pivotally connected to the base frame and selectively set between a horizontal operative position and a vertical non-operative position, and a positioning structure. The positioning structure includes a stop member forced out of the treadbase by a spring member for stopping against the stop member of the base frame so as to hold the treadbase in the non-operative position, a control block operable to move a pull slider to pull the stop member of the positioning structure away from the stop member of the base frame through a link for enabling the treadbase to be turned from the non-operative position to the operative position.
