High optical quality of potassium terbium fluoride, KTb3F10 (KTF), crystals are characterized and tested for extinction at high laser powers up to 400 W. The measured Verdet constant of the crystal is comparable with that of terbium gallium garnet (TGG) crystal. KTF crystals show high extinction at high incident power and an order of magnitude smaller absorption with respect to TGG, making it a superior crystal for high laser power applications.