A foldable treadmill includes a base frame, a front frame pivotally mounted on the base frame, a rear frame pivotally mounted on the front frame, a movable frame pivotally mounted on the front frame and slidably mounted on the base frame, a driver mounted on the front frame and provided with a retractable inner tube, and a connecting bracket secured on the movable frame and pivotally mounted on the inner tube of the driver. Thus, the rear frame is folded when not in use to reduce the whole volume of the treadmill efficiently. In addition, the front end of the rear frame is lifted to change the inclined angle of the rear frame, so that the user can practice running or walking on a slope.