  Factorization theorem plays the central role at high energy colliders to
study standard model and beyond standard model physics. The proof of
factorization theorem is given by Collins, Soper and Sterman to all orders in
perturbation theory by using diagrammatic approach. One might wonder if one can
obtain the proof of factorization theorem through symmetry considerations at
the lagrangian level. In this paper we provide such a proof.
