We studied effects of heat treatment on the magnetic transition and the magnetocaloric effects (MCEs) of Mn 1þ As 1Àx Sb x with x ¼ 0:1 and 0.25. It is found that sintering at an appropriate temperature gives a sharp magnetic transition and hence giant MCEs for Mn 1þ As 1Àx Sb x . In the bulk samples slowly cooled from a liquid state, free Sb was precipitated, which gives rise to compositional deviation. Quenching from a liquid state suppresses the precipitation of Sb. Subsequent annealing is effective to improve homogeneity, as a result, the sample undergoes a sharp magnetic transition. The optimum conditions of heat treatments of the present system for a sharp magnetic transition are discussed.