Lithium insertion behaviour of Na1+xV3O8 was studied and compared with that of Li1-xV3O8. The electrode potential and structural changes upon lithiation showed the presence of the primary single phase for 0 < x < 2.8 and the consecutive multi phases for 2.8 < x < 3.0 in LixNa1.2V3O8. Larger interlayer distance in Na1+xV3O8 contributed to a relaxation of interaction between inserted Li+ ions and did not improve a diffusivity of Li+ ion, compared with Li1+xV3O8. Lithium insertion and extraction was a reversible reaction.