A cylinder structure of an internal combustion engine contains: a body, a titanium plating layer, and two shells. The body is die casted and includes a combustion chamber surrounded by a peripheral fence and a cylinder head, a gas inlet and a gas outlet which are defined on two sides of the combustion chamber respectively, and the body includes two gas purge units, the two gas purge units have a first purge orifice and a second purge orifice. The titanium plating layer is located on a second internal fringe of the peripheral fence of the body. Each of the two shells covers each gas purge unit so as to close the first purge orifice and the second purge orifice and to define a gas conduit among the first purge orifice, the second purge orifice, and the peripheral fence.