Preservation of Integrability when conformal field theory is deformed by perturbation along RG trajectory is discussed. The deformed theories are realized by massive theories and described by S-matrix under the appropriate bootstrap assumptions. Some deformations of minimal models are investigated and S-matrices are calculated only to yield all particles. Affine extensions of Toda field theory based on semi-simple Lie algebras are reviewed and we define the mass matrix and 3-point coupling.