Solid-state phase transformation of BaB2O4 during the isothermal annealing process for both beta to alpha and alpha to beta were investigated using a platinum crucible. For the beta-phase crystal at the alpha-phase stable temperature (> 925 degrees C), the beta phase transforms to the alpha phase perfectly below the melting temperature of 1100 degrees C. Meanwhile, for the alpha-phase crystal at the beta-phase stable temperature (< 925 degrees C), the alpha phase transforms to the beta phase perfectly above 800 degrees C. There is some difference in phase transformation behaviour between bulk-shape crystals and the powder, caused by thermal stress.