Ternary Sm 55 Al 25 Co 20 alloy has been cast into full glassy rod up to at least 4 mm in diameter. Compared with the previously reported quinary Sm-based bulk metallic glass (BMG), the glass-forming ability (GFA) and the thermal stability ability of the ternary Sm 55 Al 25 Co 20 BMG are obviously enhanced. The origin of the Sm 55 Al 25 Co 20 alloy with less components exhibiting higher GFA is discussed, and it is demonstrated that the large negative mixing enthalpy is predominantly responsible for this feature of Sm 55 Al 25 Co 20 alloy.