The concept of fuel-air mixture stratification is evaluated for a single cylinder, 200-cc CNG-fuelled SI engine with port gas injection (PGI). A detailed study based on three-dimensional computational fluid dynamics (CFD) modeling has been reported. Fuel-air stratification was observed in case of PGI compared to premixed fuel-air mixture formed by a conventional gas carburetor system. Overall stratification of more than 15% was observed for PGI between the rich and lean zones in the combustion chamber compared to less than 1% in case of premixed gas carburetor. It was observed that the gas injection location, direction, timing, duration and injection pressure have a significant effect on stratification pattern.