Abstract: Based on the force acting on the wind-blown sedimentover the slope, a formula for the threshold velocity isderived. The threshold wind velocity for different slope angles in thesame wind direction andfor different wind directionsat same slope angle are calculated. They are compared with the threshold wind velocityat 0o slope angle. The results show that in the same wind direction, thethreshold wind velocity over a slope is smaller thanthat on a plane. The threshold wind velocity on a slopevaries with the wind direction. When the slope angle is zero,the formula is identical to the Bagnold formula.