In order to study about dynamic transformation phenomenon, Fe-6Ni-0.1C alloy was hot-deformed in uniaxial compression using thermo-mechanical simulator at various temperatures ranging from 600 to 1000 °C at various strain rates from 0.001 to 1 s^-1 after austenitization. As the value of Zener-Hollomon (Z) parameter increased, softening of the stress from the empirically expected value, which was extrapolated from stresses deformed at low Z value, was observed through systematical analysis of peak stresses. It suggested that this softening phenomenon was attributed to the dynamic transformation, since ferrite is softer than austenite at elevated temperature. The microstructural observation also supported that ferritic transformation occurred during compressive deformation. Even above Ae₃ temperature the softening of the peak stress of austenite was still observed, which implied that dynamic ferritic transformation might occur above Ae₃ temperature.