IOR and EOR methods are the standard ways for increasing production from reservoirs due to overcoming needs for hydrocarbon around the world. Gas injection is a widely used method in Iran for increasing production from oil reservoirs. At the time of writing this thesis, 24 Iranian oil reservoirs are at the priority of gas injection which the time of injection in 16 reservoirs have passed. Delaying gas Injection will lead to losing over 2.3 MMMSTB oil recovery. In this thesis, gas injection in a fractured reservoir which containing volatile oil was simulated by a compositional model and a discrete fracture model. Matlab Reservoir Simulation Toolbox was used as the simulator. The petrophysical Model was extracted from the SAIGUP project. Volatile Oil and reservoir fluid properties were obtained from SPE 5 Comparative Projects. The influence of different gas injection scenarios and delaying gas injection was investigated due to increasing recovery and improving GOR at this reservoir. Dry gas, Wet Gas, CO2, and Nitrogen were injected as full pressure maintenance from the start and partial maintenance after 700 days of production. For better comparison, another case without injection was added. Different situations were simulated for six years except for Dry gas, which was injected for 15 years. Delaying gas injection cause a higher pressure drop. For example, reaching to initial bubble point pressure occurred 300 days earlier in partial dry gas pressure maintenance. In the end, Nitrogen injection has the highest, and CO2 has the lowest pressure maintaining in the reservoir. Although, the production of heavier components was more senior in CO2 injection. Pressure maintenance improved GOR in 3 from 10 production wells.