Gas springs have been widely used in motor vehicles as well as in most areas of industry. Instead of coil springs, these gas springs are easily opreated to open(extension process) or close (compression process) the doors because $N_2$ gas with high pressure and oil are charged in tube. Most of manufacturers are using the trial ＆ error method in order to decide its specification(reaction force, damping force), which tends to waste time and money. Therefore, gas springs have been improved by properly changing the control pressure of $N_2$ Gas with its mounting location and weight to maximize its effect and to minimize its space. Although it has been researched on damping structure to minimize impact which is applied to vehicle when its back door is fully opened, the characteristics of damping structure are not known clearly. There(ore, this paper will not only clearly define the effect of important factors(open ＆ close force)for gas springs through theoretical analysis but also provide optimum design specification through development of program to avoid traditional method of specification determination such as the trail It error method which is widely used in whole industries including automotive industry.