A locking device to lock a collapsible treadmill deck in a folded position includes a telescopic tube and a lock. The telescopic tube includes an outside tube and an inside tube. The outside tube has a though hole. The inside tube is telescopically mounted in the outside tube and has multiple positioning holes corresponding to the through hole. The lock is attached to the outside tube, aligned with the through hole and includes a mounting bracket, an adjusting knob and a detent assembly. The adjusting knob is rotatably and retractably mounted on the mounting bracket. The detent assembly includes a rotating shaft with a locking pin movably mounted in the mounting bracket and connected to the adjusting knob. Therefore, the locking pin will be pulled into one of the positioning holes to engage the inside tube by pushing the adjusting knob toward the mounting bracket.