A difficulty encountered when using theoretical component mode synthesis method with purely experimental data is that mass matrix and stiffness matrix are necessary in order to take the residual inertia relief attachment modes into account. Yet the mass matrix and stiffness matrix of a substructure are not always available from an experimental approach. To overcome this difficulty, the approximated expressions for the residual mass matrix and residual stiffness matrix are proposed in which the mass matrix and stiffness matrix are not needed. An improved CMS formulation is then developed for the case when taking the experimental data into account. It is based on the same formulation normally used when using analytical data. However, no need exists anymore to have the mass matrix and stiffness matrix of the substructures at the same time accounting for the residual inertia relief attachment modes. The accuracy and efficiency of the proposed CMS method are proved by a numerical example.