Straight boron nitride nanotubes (BNNTs) with pure hexagonal phase were conveniently prepared by heating the mixture of Mg(BO2)(2)center dot H2O, NH4Cl, NaN3 and Mg powder in an autoclave at 600 degrees C for 20-60 h. These BNNTs had diameters mainly ranging 30-300 nm and lengths up to similar to 5 mu m, and a majority of them had at least one closed end. Besides the traditional end tips, additional cone-like tips were frequently found to be attached on the BNNTs. The effects of temperature, reactants and the possible mechanism of the catalytic formation of the BNNTs are discussed. (C) 2007 Elsevier B.V. All rights reserved.