Variations in Mg and Ca stable isotope ratios provide constraints on a variety of geological and biogeochemical processes. Recent improvements in thermal and plasma mass spectrometry have allowed the precise measurement of small differences in Ca and Mg isotopic compositions in CaCO₃, and consequently, their isotopic fractionation has been considered to have complementary information regarding marine temperature reconstructed from conventional proxies. These new tools can provide an opportunity to refine a biological control during biomineralization processes, which is critical for improvement of biological proxies. Studies on isotopic fractionation of marine CaCO₃ have been also performed to investigate past changes in marine Ca and Mg budgets that are controlled, for example, by continental weathering, mid-ocean ridge spreading rate, carbonate sedimentation, and dolomitization. Therefore, marine Ca and Mg isotope ratios play important roles to understand global Ca and Mg cycles and it is suggested that these cycles are likely related to climate change. This article reviews present knowledge and the potential of Ca and Mg isotope ratios in biogenic and inorganic CaCO₃ for reconstructing seawater temperature and isotope ratios in the oceans over geologic past.