The transition energies (2s2S1/2 - 2p2P1/2,s/2) of lithium-like systems with nuclear charge from Z = 11-20are calculated by using a full-core pins correlation method. Relativistic and mass-polarization effects on theenergies are included as the first-order perturbation corrections. The quantum-electrodynamics contributions tothe transition energy are evaluated by using effective nuclear charge. Our results are in excellent agreement withprevious theoretical and experimental data available in the literature.