The variation of retained austenite content in heat treated steels is studied here by an X-ray diffraction technique. It is found that the retained austenite content in steel samples studied increases with the austenitizing temperature and decreases slowly with the tempering temperature up to 177 °C. Above this temperature, it decreases very rapidly until at the tempering temperature of 260 °C it becomes completely zero. The effects of surface preparation on the results of X-ray diffraction measurements are also demonstrated.