The effects of carbon and/or alkaline earth elements Ca and Sr on the grain refinement and tensile properties of the AZ31 alloy have been investigated in the present study. A significant grain refining efficiency could be obtained for the AZ31 alloy modified by carbon inoculation and the grain refining efficiency could be further improved by the combination of 0.2 mass%C and alkaline earth elements of 0.2 mass%Ca or 0.2 mass%Sr. Compared to the AZ31 alloy without any treatment, the tensile properties of the AZ31 alloy were remarkably improved after being modified by the combination of carbon and a little addition of alkaline earth elements. The ultimate tensile strength and elongation to failure were improved by about 20% and 40%, respectively. After being refined either by 0.2 mass%C or by the combination of 0.2 mass%C and a little addition of alkaline earth elements (0.2 mass%Ca or 0.2 mass%Sr), the main fracture mechanism was changed from cleavage mode with large cleavage planes for the unrefined AZ31 alloy to mixed mode of cleavage and quasi-cleavage fracture. The fracture surfaces were almost composed of small cleavage planes with thin river patterns and quasi-cleavage planes with small dimples and severe plastic deformation.