This paper sets forth the results of tank experiments on the hydrodynamical force upon a circular cylindor in uniform flow, which was measured directly and dynamically by the use of a specially designed strain gauge-type dynamometer. As the aim of experiments was to study the dynamical behavior of tall stack under the action of wind, they were carried out over the range covering the critical Reynolds number, R =0.86×10⁵ According to the test results, the drag force is stable in spite of the fluctuation of the lateral force and the Strouhal number referring to the lateral force coincides with the Relf's results which were obtained from the fluctuation in the wake flow. The drag coefficient C_D , the ratio of single amplitude of fluctuating lateral force to drag force, C_R . and the Strouhal number S versus the Reynolds number R are shown by the curve with mark TUET in Fig. 1. From these results it is supposed that beyond the critical Reynolds number R ≈2×10⁵ a realistic value of C_L is in the order of 0.1. This value, as compared with the value C_L ≈1.0 suggested by Den Hartog and Fung, is considerably smaller, and is of the same order of C_L _??_0.20 by Ozker and Smith.