By utilizing propeller-rudder system with the absence of the ship body, the author conducted experiments concerning characteristics of a rudder behind the propeller in oblique flows in order to make clear flow-straightening effect of the propeller itself. The tests were performed by varying drift angle of the propeller axis, rudder angle and propeller loading systematically. Through these tests, propeller flow direction at the rudder position was obtained from the rudder direction in which normal force did not act on the rudder. The results were represented in the form of a coefficient of flow-straightening effect. Principal conclusions obtained through the studies are as follows : 1) It was confirmed from experiments that the coefficient of flow-straightening effect is constant regardless of drift angle. 2) The coefficient estimated from direction of the resultant velocity from drifting velocity of a propeller and increased axial velocity of the slipstream agrees with the actually measured value fairly well. 3) Influence of propeller loading on the effect is remarkable. This fact is related with the problem of model-ship correlation for maneuverability. 4) This paper suggests an idea which may be called flow-straightening effect identity as a method to obtain the effect of a ship's hull using that of the propeller itself.