In this work, the polymorphic phase transformations of KNO3 were studied by thermo-Raman spectroscopy in a dynamical thermal process with a heating rate of 5 degrees C min(-1) from 30 degrees C to 170 degrees C and then cooling down to 30 degrees C at the same rate. Three distinct phase transformations were detected. The characteristic spectrum for each phase was identified. The nature of each phase transformation was investigated in detail through the studies on spectral variation, intensity, intensity variation and shift in band position. In addition, the intensity variation of the nu(1) mode at 1042 cm(-1) clearly revealed thermal hysteresis involved in the phase transformation of KNO3. Furthermore, the thermal hysteresis of each phase was also obtained from the nu(3) mode in the range from 1341 to 1353 cm(-1).