The present invention relates to a pressure reactive piston for a reciprocating type internal combustion engine, particularly provides a pressure reactive piston for an internal combustion engine including an axially directed central bore formed within a piston ring portion of the piston, which houses a slidably mounted crown which cooperates with the central bore to define a gas chamber which isclosed off from the environment by means of a flexible gas seal interposed between the crown and the ring portion of the piston. The pressure reactive piston may play the benefit of low compression rate and high compression rate in a single piston.