To characterize the creep deformation mechanism of 2.25Cr-1Mo Steel, stress change tests were conducted during creep tests. In this study it was confirmed that the dislocation behavior during the creep tests were in viscous manner, because no instantaneous plastic strain was observed at stress increments and transient backward creep behavior was observed after stress reduction same as Modified 9Cr-1Mo Steel. Mo-bilities of dislocation were evaluated by observed backward creep behavior after stress reductions. Internal stresses were evaluated by the changes of creep strain rate in stress increments. These values were almost stable during creep deformation. Mobile dislocation densities were evaluated with the estimated mobilities of dislocation and the changes of creep strain rate in stress increments. Variation of evaluated mobile dislocation densities with creep strain showed the same tendency as variation of creep strain rate. Therefore mobile dislocation density is the dominant factor that influences the creep strain rate in creep deformation of this steel. The difference of estimated mobility of dislocation between 2.25Cr-1Mo Steel and Modified 9Cr-1Mo Steel was considered to be caused by the different amount of solute Mo between these steels.
