Crystalline Co3O4-Al2O3 composite oxide was prepared by microemulsion chemical method and the electrochemical properties as anode material for lithium ion batteries were studied. The results show that the particle sizes of Co3O4 can be changed by amorphous Al2O3, but the crystal form of Co3O4 can’t be changed. The electrochemical properties of Co3O4·xAl2O3 as anode material for lithium ion batteries are sensitive to its particle sizes and the Co3O4·0.1Al2O3sample exhibits excellent electrochemical performance with the average particle size of about 30 nm: from the 1th cycle to the 20th cycle, the discharge capacity decreases from 929 mAh/g to 910 mAh/g, and the average decay of charge capacity is 0.01% per cycle