Let x : (M, F ) →֒ (V n+1, F ) be a simply connected hypersurface in a Minkowski space (V n+1, F ). In this paper, using the Gauss formula of Chern connection on Finsler submanifolds, we shall prove that if x(p) is normal to Tp(M)(∀p ∈ M), then M with the induced metric is isometric to the standard Euclidean sphere.