When discussing the performance of an actual ship based on the model experiment for maneuverability, the model-ship correlation is basically significant. Many correlative studies show that the steering quality indices obtained from Z-maneuver tests of actual ships considerably differ from those of model ships. This paper introduces a new procedure to analyze maneuvering motions. In this new procedure, the response behaviour of a ship to rudder force is examined instead of the response to steering rudder angle in the conventional method. On the assumption that the discrepancy relates largely to the difference in rudder effectiveness, the author have suggested a new method to predict the scale effect of the steering quality indices through the procedure above-mentioned. This method will allow us to predict the indices of an actual ship by measuring rudder force during Z-maneuver test of the model ship. The steering quality indices of a full size ship and the 30 meter long large size model ship were predicted based on the results of the test conducted for the model ship at the Experimental Tank by this method. As the result, the values predicted by this method are fairly in agreement with those measured for the large size model and the actual ship. This fact reveals that the suggested method will be reasonable.