The formation energies of many kinds of defects for oxygen-deficient LiMn2O4, including both oxygen-vacancy types and metal-interstitial types, have been determined by first principles plane-wave pseudopotential calculations. Two kinds of metal-excess defects show the lowest formation energies. They were found to be more stable than the simple oxygen vacancy that exhibits the lowest formation energy among oxygen-vacancy-type defects. In both of the metal-excess defects with the lowest formation energies, excess Li occupies an empty 16c position of spinel. Excess Mn is present either at the 8a position, substituting for Li, or at the empty 16c position, moving one neighboring Li into the next 16c position. The results are consistent with experimental results of density measurements, diffusion experiments, and decomposition reactions at high temperatures. Detailed examination of the electronic structures revealed that formal charges of both interstitial and substituted Mn are +2.