Abstract: An enclosed hydraulic cylinder which may be used as a tension buffer, mainly consists of a body casing having an integrally formed sealed inner passage and a piston stem. The body casing is internally divided into a pressure chamber, a buffer chamber, and a regulation chamber between the pressure chamber and the buffer chamber. When the body casing and the piston stem are subjected to external pulling force in reverse direction, fluid filled in the pressure chamber, being compressed by air coming from an air intake chamber communicable with the pressure chamber and by the piston stem, flows through a flow-limit disposed near outer end of the pressure chamber and into the sealed inner passage before it enters the regulation chamber and thereby adjusts the inner pressure of the cylinder. The buffer chamber adjoins the regulation chamber and the room thereof can automatically adjust to relieve the hydraulic pressure in the regulation chamber and prevent the body casing from excess hydraulic pressure.