Many of oil reservoirs have dealt with operational problems due to probability of asphaltene deposition as a consequence of asphaltene precipitation during natural production and gas injection into the reservoir. So the prediction of asphaltene precipitation is very important and many equations of state (such as Ping Robinson (PR) and Soave–Redlich–Kwong (SRK)) are used for this reason. These common equations are suitable for non-polar components and a modification is necessary to use them for prediction of asphaltene precipitation because of the polar nature of asphaltene compounds. In this study, the SRK equation of state was modified by deriving a new equation for calculating b-parameter (co-volume parameter); and this modified SRK equation of state was used to model asphaltene precipitation. Finally asphaltene precipitation during natural depletion and first stage gas injection process (in different concentrations), was monitored at reservoir temperature and various pressures. The experimental results show a good match with the modified SRK equation of state.