A disposable syringe after one time usage includes an outer tube having an inner teeth set formed on the inner wall of the outer tube and an inner tube having an outer teeth set formed on the outer wall of the inner tube and a safety guide. The inner teeth set of the outer tube meshes with the outer teeth set of the inner tube. The single direction design of the teeth sets allows the outer tube to be pulled away from the inner tube, but not to be pushed back. The pulling force of the outer tube will pull the safety guide to form a damaged groove and the outer tube is adapted to enclose a needle so that the needle is not exposed outwardly, which prevents the needle from poking people accidentally. The syringe is unable to be reused.