The ferroelectrical behaviour and thermal properties of a thin layer of KNO3 have been investigated mainly in its metastable phase III over the temperature range 145-90 degrees C on cooling. The D-E hysteresis loop along the c axis has been observed by a modified Sawyer-Tower circuit using a 50 Hz a.c. electric field. The spontaneous polarization, coercive field and loop area of the ferroelectric hysteresis loop have been investigated as temperature dependent parameters. High accuracy differential thermal analysis (DTA) has also been used here to measure the thermodynamic and kinetic data of the phase transition phenomenon. The analysis shows that the heat of transformation is Delta H= 7.5 J g(-1) and there is a thermal hysteresis Delta T-h Of 10 degrees C. The results are discussed according to the nature of the ferroelectricity in phase III of KNO3.