Two kinds of Ca[B6O9(OH)(2)] center dot 3H(2)O morphologies, a nanoflake and an ellipsoidal-like nanostructure, have been prepared by an ultrasound-assisted phase transformation of ulexite in H3BO3 aqueous solution. All samples were characterized by XRD, FT-IR, TG-DTA, SEM and chemical analysis. The possible formation mechanism of calcium borate Ca[B6O9(OH)(2)] center dot 3H(2)O was proposed. Furthermore, the flame retardant properties of synthesized Ca[B6O9(OH)(2)] center dot 3H(2)O nanostructures were investigated by a thermal analysis method, demonstrating that the nanoflake sample with smaller size had better behavior than others. Molar enthalpies of solution of prepared Ca[B6O9(OH)(2)] center dot 3H(2)O samples were different because of their different morphologies and sizes. Through an appropriate thermochemical cycle, their standard molar enthalpies of formation were obtained, which decreased in turn from nanoflake to ellipsoidal-like nanostructure and then to flower-like microstructure, showing that their stabilities increased gradually with the increase of Ca[B6O9(OH)(2)] center dot 3H(2)O sizes. (C) 2013 Elsevier B.V. All rights reserved.