Abstract: In order to properly decide transverse grads of runway, this paper researches the directional stability of aircraft that runs on the runway with two transverse slopes.On the basis of analyzing the forces acting on the aircraft, this paper sets up the motion equation of the aircraft, also puts forward the calculation method of the sideslip angle, using parameter of a kind of aircraft, gets the relation between the aircraft directional stability and the value of runway grads.Resultsshow that transverse grades make very limited influence on the aircraft, when the value of grads is in the range of current criteria (0.8%~1.2%).Even the value of grads reaches 4%, this kind of aircraft keeps its directional stability well.Conclusionsare considered in new criteria.