This paper presents a new method for the measurement of surface stress. By the use of test pieces plated with copper, the stress is measured by observing the flecks on the surface produced by cyclic stress. The variation in the appearance of flecks which is due to the change of load is very sensitive at a proper load, so an accurate determination of the surface stress is feasible.The stress concentration factor in torsion is obtained experimentally by the method described for circular shafts with a circumferential notch in each. The results are compared with theoretical values, calculated from two different formulas previously derived by the author and Sonntag, respectively. It is confirmed that the experimental results obtained here are in good agreement with those calculated by the author's formula.