There is provided a composition of refrigerant mixtures for low back pressure condition, comprising isobutane, 1,1-difluoroethane and, optionally, additive selected from the group consisting of carbon oxide, trifluoromethyl iodide and mixture thereof. The composition according to the present invention is environment-friendly and can be directly drop in the conventional refrigerator system adopting CFC-12 refrigerant without any systemic change.