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The point defect concentrations of L12-Al3X(Sc, Zr, Er) were systematically investigated using the first-principle calculations with thermodynamics approach. The results show that the constitutional point defects of off-stoichiometric L12-Al3X(Sc, Zr, Er) prefer to occur in X sublattice, that is X anti-site in X-rich alloy and X vacancy in Al-rich alloy, respectively. And Al anti-site also has a high density in Al-rich L12-Al3X(Sc, Er). It is found that the point defect concentrations of stoichiometric L12-Al3X(Sc, Zr) follow in the sequence as: Al vacancies (VAl) > X vacancies (VX) > X anti-sites (XAl) > Al anti-sites (AlX). The point defect concentration of stoichio-metric Al3Er is similar to that of L12-Al3X(Sc, Zr). The result suggests that the Al vacancy (VAl) is a dominant point defect in L12-Al3X(Sc, Zr, Er). A simple parameter HVX–HVAl can be used for a rough estimation of the point defect concentrations in L12-Al3X structure. Some rules of point defect concentrations for L12-Al3X(Sc, Zr, Er) are also revealed.