The nonrelativistic energies of the core-excited (n = 3, 4, 5 and 6) states for the lithium isoelectronic sequence from Li I to O VI are calculated with a full core plus correlation method using multiconfiguration wavefunctions. Relativistic and mass-polarization effects on the energy are evaluated at the first-order perturbation theory by using the Pauli-Breit operators. Our results are in close agreement with experiment and are better than other theoretical results.