Fe–6mass%Ni–(0.0008∼0.29)mass%C alloys were hot-deformed in torsion at 600–720°C (above the cooling transformation start temperatures A_r3 ) after austenitization. An in-situ X-ray diffraction study re-vealedthat γ→α transformation occurred during deformation in a wide range of condition, even above A₃^p(paraequilibrium γ→α transformation temperature). Corresponding to this transformation, apparent decrease in deformation stress from that expected for austenite was observed. Microstructural study of the specimens quenched after the deformation showed that a large amount of fine-grained ferrite was formed due to the deformation. The analysis of deformation stress and chemical driving-force of the transformation indicated that the transformation occurred in order to reduce the total energy of deformed material since the deformation of energy of α was revealed to be considerably smaller than that of γ and the amount of deformation energy saved by the transformation was shown to be much greater than the chemical energy consumed by the transformation at the tested temperatures.