Steady state creep rates and friction stresses based on the strain dip test were measured at 1 000°C with a series of carbon free 25Cr-35Ni austenitic steels, and the solid solution strengthening with the additions of groups IV, V and VI elements to the steel was discussed in terms of friction stress. The steady state creep rate of the steel decreased remarkably with the increase of Mo, W, Nb, Ta, Zr and Hf contents, but slightly decreased with the additions of Cr and Ti. Friction stress increased remarkably with the additions of Mo, W, Nb, Ta, Zr and Hf, and, therefore, the changes in the steady state creep rate with the additions of these elements was found to be quite similar to that in the effective stress (=applied stress minus friction stress). A single linear relation was found to exist between logarithms of the steady state creep rates and the effective stresses of all the steels independent of the kinds and the amounts of additional solute elements. The results conclusively demonstrate that the solid solution strengthening at the high temperature due to the additions of Cr, Mo, W, Nb, Ta, Ti, Zr and Hf to the carbon free 25Cr-35Ni austenitic steel is mainly attributed to the increase in the friction stress.