This study presents the stress safety analysis of a cylinder, which is manufactured by a tempered ASTM D2, tempered SM45C and normalized SM45C materials, respectively. The inner diameter of three cylinder models are 85mm, 95mm, and 11 Omm and the total length of a cylinder is 2,365mm for a high pressure injection molding machine. The FEM computed results show that the inner diameter of 85mm with a thick thickness of 62.5mm may produce the injection pressure of 325MPa and the inner diameter of 110mm with 50mm thickness reduces up to the injection pressure of 220MPa because of a reduced thickness of a cylinder. These injection pressures are enough for a high pressure injection molding machine assembled by ASTM D2 cylinder. And also, an injection cylinder manufactured by a tempered SM45C material in which is low priee may produce 225MPa injection molding pressure and this may sufficiently endure stress safety compared to that of ASTM D2 cylinder material. Thus, this study recommends that tempered SM45C cylinder is appropriated for a mild injection molding machine as an alternative cylinder material when the safety strength and low prices are considered. But the normalized SM45C cylinder material does not meet a stress safety of yield strength in general.