Many studies have shown that microstructural changes in austenite usually take place during hot deformation and those deformed structures can affect the transformation kinetics to ferrite and the final microstructure. The change of dislocation density during deformation is one of the most important microstructural changes. However, a direct evaluation of the dislocation density of the hot deformed austenite is difficult because austenite in low carbon steels transforms to ferrite or to martensite during subsequent cooling to room temperature. Even though, a direct measurement of the dislocation density in the austenite may be possible with TEM in highly alloyed steels, where austenite is stable even at room temperature, the observation is generally restricted to a small area. One of the prospective methods to evaluate the dislocation density in materials is by using the flow stress and mathematical formulae that can link the flow stress to dislocation density in materials. Yoshie et al. and Senuma et al. reported that the flow curves of austenite could be described by mathematical formulae taking into account the dislocation density. Present authors think their works offer a useful tools for evaluating the dislocation density in hot deformed materials. However, systematical investigations to assess the dislocation density in hot deformed materials from the measured flow curves have not been available. Furthermore, the plausibility of the evaluated parameters in mathematical formulae has not been fully discussed yet. An attempt to propose a procedure for evaluating the dislocation density using flow curves of material, which shows a dynamic recovery behavior during hot deformation, is made in this study.