The effect of different Cl-containing additives on the gas phase reaction of oxidative coupling of methane (OCM) at elevated pressures has been investigated. NH4Cl was found to have the best beneficial effect to improve C-2 selectivity; however, CH3Cl and C2H4Cl2 have almost no effect on gas phase reaction of OCM at elevated pressures. We conclude that Cl- and NH3 in NH4Cl or HCl played the key roles in improving the selectivity to C-2. Although the addition of only NH3 improved the C-2 selectivity very little, NH3 favored the formation of CI radical in the presence of HCl. The results also imply that Cl atoms in organic compounds have a different effect from Cl-ion in inorganic compounds on gas phase reactions of OCM.