If the smooth vector fields X1,…,Xm and their commutators span tangent space at every point in Ω⊆RN for any fixed m≤N, then we establish full interior regularity theory of quasi-linear equations ∑i=1mXi⁎Ai(X1u,…,Xmu)=0 with p-Laplacian type growth condition. In other words, show that a weak solution equation is locally C1,α.