From economical view, natural gas is frugal fuel by low cost and with abundant resources, on the other hand this combustion pollution are lower than other common fossil fuels such as gasoline and diesel. A major issue of natural gas fuel using in cars, is the storing problem. Compressed natural gas (CNG) cylinders are used for storage of fuel in high pressure in cars with CNG. These cylinders is divided into four categories according to the manufacturer's material that including: all-metal, metal liner hoop wrapped, metal liner fully wrapped and all-composite. All–steel cylinders with 91.7 percent usage are the most common type of cylinders and history of these cylinders goes back to 1974. There are three known methods to produce this type of cylinders so that their difference is in the raw material. Safety is one of the important issues in design and manufacturing of such cylinders. All–metal cylinders are using more well-known technology with respect to the other types of cylinders and therefore have more capability to safe performance. According to the importance of safety and decrease concern of gaslight car’s passengers, because of incident due to CNG cylinders impact with barriers especially in car accidents, it is necessary to investigate cylinder impact conditions before use. High cost and risk of empirical tests make the use of numerical methods inevitable. In this thesis damage mechanics approach is used to investigate the effect of crash and damage caused by impact in CNG under pressure steel cylinder. The CSA standard in CNG cylinders is used as a damage detection criterion and cylinders ability to reuse. Simulation of cylinder failures caused by car accident and drop is done by using Johnson-Cook damage model which is one of the efficient models in impact problems. Accomplished simulations are carried out in different impact directions, and effect of cylinder internal pressure, collision velocity and fall height are analyzed. Also failures due to collision for various situations are discussed. These investigations for different cases including crash and drop tests show that the maximum damage created in case of vertical impact and by changing direction from vertical to horizontal resultant damage will be decreased. Also by eliminating failed elements and comparing damage depth caused by collision with CSA standard, it is observed that in most cases of vertical accident and drop tests, cylinders have been damaged and lose its ability to use, while in horizontal impact cases cylinder is intact or can be reused after repairing. The results show that in collision process the cylinder’s rear wall and the front hemisphere of cylinder have more damaged and are the critical areas in the horizontal and vertical collisions, respectively. For a specific impact direction in lower cylinder internal pressure, higher collision velocity and altitude of fall damage will be more. The resulted diagrams indicate that damaged area of the cylinders predominately are under compression and endure large plastic deformation. The low difference between the results by various meshes shows that this solution does not depend on the mesh size. Therefore, this damage model is insensitive to meshing in the various impact cases. Keywords: All-steel CNG cylinder, Damage Mechanics, fracture, Impact, Jahnson-Cook model