To improve structural and electrochemical properties of LiMn2O4 thin films, cobalt ions were substituted for manganese ions by a co-sputtering method. The annealed films had the spinel-phase, which had a preferred orientation of (111). Compositions of the films were nearly the stoichiometric LiMn2O4 and LiCo0.26Mn1.74O4, respectively. In the electrochemical test, the discharge capacity of the LiCo0.26Mn1.74O4 film was lower than that of the LiMn2O4 film. However, cyclability of the LiCo0.26Mn1.74O4 film was better than that of the LiMn2O4 film. The cobalt ions effectively improved the structural and electrochemical properties of the spinel-phase LiMn2O4 thin films.