CO 2 injection into impermeable underground structures is done in two general ways: The first is the storage of CO2 gas for controlling greenhouse gas emissions. In 1990, the topic of the geological storage of CO2 gas was proposed to control greenhouse gas emissions. The second is the injection of carbon dioxide gas to increase the recovery and exploitation of oilfields. By injecting the gas into the reduced hydrocarbon reservoirs, increasing the internal pressure of reservoir, under a particular condition, the recovery of hydrocarbon is incresed too. However, in gas injection, both in order to storage and to increase the production of hydrocarbons from a particular reservoir, the most important issue is to ensure the stability and preservation of the integrity of the rocks as the most important impermeable part of reservoir during the injection operation and even after the end of the operation. By injection of gas into the reservoir,, the stable properties of the rock and the pattern of initial stress in the region under the injection are changed and cause to faults and seizure events. In this study, the effect of gas injection inside the hypothetical reservoir under some condition was considered. The aim is to simulate a reservoir under specific conditions in the case of gas injection in order to considering the stability of cap rock, activity of existing faults, probability of ceration of new faults and their influence on integrity of cap rock. This simulation is done by applying the FLAC3D code.