In this thesis, a new formulation of the enhanced automated multilevel substructuring method (EAMLS) is presented. To improve the computational efficiency, the major problems which required large computation time and computer memory are inspected and dealt with effectively. In the new formulations, an extended root substructure is defined by assembling the reduced higher level substructures, and reduced model is projected on a new refined subspace derived by extended root substructure. Then, the residual substructural modes correction computed by residual flexibility matrices of bottom substructures is performed only for the mass matrix, which gives rise to reduction of computational cost and required memory. In addition, the reduced mass and stiffness matrices are computed by submatrix level instead of global matrix. The solution accuracy and computational efficiency of the new formulation are demonstrated through several large FE models.