Abstract: The thermal features of nacre aragonite from bivalve shells of Pinctada martensii are analyzed by using differential scanning calorimetry. It is observed that the thermal transformation of nacre aragonite to calcite is exothermal at about 445℃, whereas the transformation of synthetic aragonite is endothermal at about 498℃. The unusual exothermal transformation of nacre aragonite to calcite is perhaps resulted from the stored excess enthalpy in nanometer-size aragonite of nacre.