A ternary Li-Mg-B alloy (64%Li, 4%Mg and 32%B, all compositions are reported as weight percent in this paper) foil was prepared and its phase structure and discharge behavior were investigated. The main phases existing in the alloy included Li5B4, Li3Mg7, LiB and Li. Coupled with LiNO3-KNO3 eutectic electrolyte and MnO2 cathode, the anode exhibited great potential in the intermediate temperature lithium batteries. At temperatures above 200°C, two steady voltage plateaus were obtained at near 3V and 2.7V, and the cell displayed an open-circuit voltage exceeding 3.2V. The experimentally measured capacity of the Li-Mg-B alloy was 5427C·g-1 at 200°C, which was higher than that of LiB alloy (70%Li). Adding magnesium to LiB alloy could lower the cost of anodes without capacity loss.