Abstract: A rotor for an eddy current retarder has a plurality of cooling fins provided around an outer surface of a drum. A transverse cross section of each of the cooling fins is rectangular or trapezoidal, and heights h of the cooling fins are equal, and the following expressions (1) to (3) are satisfied. Since this rotor is excellent in cooling ability, this is suitable as a rotor for a retarder of a large vehicle. 6 mm≦d≦11 mm  (1) 25°≦&thgr;≦55°  (2) 20 mm≦H≦200 mm  (3) wherein d is distance between bottoms of the cooling fins, &thgr; is a fin angle in a longitudinal direction of the cooling fin with respect to a rotational direction of the rotor, H=2h2/(t1+t2), t1 is thickness of the bottom of the cooling fin, and t2 is thickness of top of the cooling fin.