Phase transformation from beta-BiNbO4 to alpha-BiNbO4 in BiNbO4 bulk samples was studied. From x-ray diffraction patterns, the transformation from beta to alpha phase of BiNbO4 could be observed by heating the bulk samples of beta-BiNbO4 from low temperatures to 700-1030 °C. Such a transformation did not occur in powder samples and in the cooling course. This phenomenon might be related with associated activation of stress and heat energy in the heating course. Differential thermal analysis, shrinkage, and dielectric properties as a function of temperature were carried out, and all the results confirmed the transformation from beta to alpha phase of BiNbO4.