Three kinds of alkenyl succinic anhydrides (ASA) with varying side chain lengths (2-octenyl, 2-dodecyl, and 2-hexadecynyl succinic anhydrides) for hardeners as epoxy curing agents are synthesized by the ene reaction. The curing effect of ASA on the mechanical tensile and impact properties of cured diglycidyl ether of bisphenol-A (DGEBA) epoxy resin is studied. It is observed that increasing the side alkenyl chain length of ASA leads to a decrease of tensile strength of the cured epoxy resin, probably due to the decreasing of cross-linking density. However, it is found that the impact properties of the casting specimens are increased as the side alkenyl chain length of the hardener increases. This is probably due to the effect of the nonpolar or London-dispersive component of the surface free energy of the ASA studied, resulting in improving of the toughness properties of the casting specimens,