The purpose of this paper is to investigate compositions on the interface between LiCoO2 and electrolyte when trimethyl phosphite TMP(i) is used as an additive in 1M LiPF6/EC+DEC electrolyte system and the thermal stability of the electrolyte as well as Li0.5CoO2 mixed with the electrolyte. The electrochemical performance of LiCoO2 electrode in the two electrolyte systems was also studied. It is found that the electrochemical performance, including capacity, cycle performance and 3.6V plateau efficiency, has been improved in the electrolyte with TMP(i) additive. FTIR analysis indicates that Li x PO y is an important surface film composition on the cathode in TMP(i) containing system. A thicker and more passivating surface layer is formed when using TMP(i) additive as an additive. The thermal stability of the cathode is substantially improved in the electrolyte containing TMP(i) additive in the system, especially the exothermic peak around 190°C, which is associated with the reaction between active surface of cathode and solvents, is obviously restrained.