Drag reduction with polymer additives was measured in a 90° triangular open channel. Also, the laser Doppler technique was employed to measure the velocity profiles in the same flow system. The amount of drag reduction depended on both Reynolds number and polymer concentration in almost the same manner as obtained in a turbulent pipe flow. In addition, a large effect of Froude number on the amount of drag reduction was observed. In supercritical flow, the values of the friction factors were almost constant over a wide range of Reynolds numbers. The results of the velocity measurements showed that the decrease of the eddy diffusivity near the free surface was very significant.