Production of anhydrous borax from borax pentahydrate was investigated in a fluidized bed calcinator. Single step calcination gives a puffed product with very low density. In order to obtain high bulk density product dehydration should be carried out at least in two stages. The most important step dominating the final bulk density is the first step. Dehydration of borax pentahydrate up to around 80% Na2B4O7 content at temperatures lower than 393 K in the first step gives high bulk density anhydrous borax at the final stage. After this critical point, thermal shock imposed on the particle has only slight effect on the final bulk density value and anhydrous borax with a bulk density higher than 0.6 g cm(-3) may easily be obtained at temperatures less than 800 K.