A treadmill includes a base assembly, a collapsible treadmill deck and a locking device that includes a telescopic tube and a latch. The telescopic tube supports the treadmill deck in a folded position and includes an inside tube and an outside tube. The outside tube has a pawl hole and a bottom end pivotally mounted to the base assembly. The inside tube is telescopically mounted in the outside tube and has an outside end pivotally mounted on the treadmill deck and multiple positioning holes aligned with the pawl hole. The latch includes a stationary bracket fastened on the outside tube and a pivot pawl pivotally mounted on the stationary bracket. The pivot pawl has an inside end extended into the pawl hole to engage one of positioning holes to interlock the inside tube with the outside tube to firmly hold the folded treadmill deck in position.