The relative stability of Watson-Crick and mismatched dNTP-template base pairs in the active site of T7 DNA polymerase (pol T7) was examined using linear-response (LRA) molecular dynamics simulations. The LRA method approximates the relative binding free energies by evaluating average electrostatic and van der Waals energies. The results of these computer simulations and the previous simulations of the human DNA polymerase P (pol) ternary complex (Florian, J.; Goodman, M. F.; Warshel, A. J. Phys. Chem. B, 2002, 106, 5739) carried out at the same theoretical level were compared. It was found that pol T7 provides a larger binding contribution to the replication fidelity than pol 0 by discriminating more effectively against mismatches that are most probable to occur in the neutral anti-anti configurations. The selectivity of the pol T7 in the binding step is largely determined by the template-dNTP interactions. These interactions are strengthened by the preorganized protein active site and the contribution of the crystallographic water molecule positioned in the minor groove of the newly forming base pair. The pol T7 active site limits the vertical displacements of dNTP base relative to the base-pair plane more effectively than the pol 3 active site. On the other hand, the pol T7 active site provides greater flexibility for the dNTP base to be displaced in the major-groove direction.