A composite of transition metal oxide Co3O4 and multiwalled carbon nanotubes (MCNTs) was applied as an anode material for sodium-ion batteries via a simply modified solid-state reaction and sonication method. The as-prepared Co3O4/MCNTs composite shows improved electrochemical performance than bare Co3O4 owing to the Co3O4/MCNTs nanostructure that benefits Na ion and electronic transport together with buffering the large volume change of Co3O4 during charging and discharging process. Additionally, the Na-storage behavior and the original capacity loss of Co3O4 based on the conversion reaction have been investigated through cyclic voltammogram, X-ray diffraction, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, and selected area electron diffraction. It is firstly demonstrated that in discharge state, Co3O4 particles initially react with Na to produce CoO and then is further reduced to nanosized Co in an amorphous sodium oxides matrix. In the charge process, Co nanoparticles were partially oxidized to Co3O4, the other part of the CoO remain leading capacity loss. These results offer new insights into the electrochemical process of transition metal-based anode materials for Na-ion batteries.