We electrochemically and crystallographically investigated the quantitative details of relationship between cycle life of Li(Mn2-xLix)O4-delta (nominally x = 0.02) spinel and oxygen deficiency delta which was precisely determined by chemical analysis. We found that capacity retention rate after the 100th cycle at 25 degrees C is linearly decreased with increasing oxygen deficiency. The changes in lattice parameter before and after 500 times cycling test were compared with the changes in potential curves. The changes in lattice parameter at the end of discharged state after the 500th cycle depended on oxygen deficiency delta. They were in good accordance with the degradation in the low voltage region of potential curve, which was strongly affected by oxygen loss and rapidly saturated within about the 100th cycle. These results implied the influence of Jahn-Teller ion Mn3+ introduced with oxygen deficiency.