Li2MO3 (M: transition metal), which has a similar layer structure to LiCoO2, is interesting for use as another layered rocksalt-type oxide cathode for Li-ion batteries. We found that Li2PtO3 in Li2MO3 system is the first full 4 V class cathode material among 5d metal compounds. We investigated the electrochemical, structural and kinetic characteristics of the 4 V region. The volumetric capacity of the 4 V plateau on discharge is comparable to that of LiCoO2. Moreover, the reversibility of the structural change in the 4 V region was confirmed by X-ray diffractometry.