Let Ω ⊆ R m be a bounded domain, X = {X 1 , X 2 , · · · , X k 0 } be a Hömander family of vector fields on Ω whose homogeneous dimension is Q. In this paper, we improve the dual inequality obtained by P. Hajlasz and P. Strzelecki in 1998, and take use of it to discuss regularity of weakly subelliptic F-harmonic maps into Riemannian manifolds with transitive isometric transformation groups.