In this paper, the thermal calcinations of ammonium tetraborate tetrahydrate (ATT) to boron oxide investigated in fluidized bed calcinatory. Single step calcinations of ATT in pure state gives a puffed product with very low bulk density and the calcination is incomplete since ATT particles completely agglomerate at temperature higher than 350 degrees C. Effect of the temperature on the bulk density and particle size distribution product obtained at the end of single step is given and compared with theoretical calculation. In order to restrict agglomeration at temperature higher than 300 degrees C, the surface of ATT has been covered with a material with a higher melting point than boric oxide. In order to obtain this high melting cover, a mixture of ammonium tetraborate tetrahydrate and Ca(OH)(2) has been calcined in fluidized bed calcinatory. During this calcinations process, quantities such as the bulk density value, particle size distribution and B2O3 content of product have been determined as a function of temperature. It was found that the boron oxide can be obtained by calcinations of ATT particle covered with Ca(OH)(2) in 90 min time intervals at a temperature range of 450 and 500 degrees C. As a result, puffed granular boron oxide of 99% purity with bulk density about 0.30 g cm(-3) has been produced using single step calcination by this new method. (c) 2006 Elsevier B.V. All rights reserved.