On a real vector space V, a Randers norm ^ F is de, ned by ^ F = ^, + ^ , , where ^,is a Euclidean norm and ^ ,is a covector. We show that the unit sphere ,in the Randers space (V,^ F) has positive ag curvature, if and only if j ^ , j^,< (5 ,p 17)=2 ,0: 43845, thus answering a problem proposed by Prof. Zhongmin Shen. Moreover, we prove that the ag curvature of ,has a universal lower bound , 4.