To improve the safety of lithium ion battery, cresyl diphenyl phosphate (CDP) is used as a flame-retardant additive in a LiPF6 based electrolyte. The electrochemical performances of LiCoO2/CDP-electrolyte/Li and Li/CDP-electrolyte/C half cells are evaluated. The thermal behaviors of Li0.5CoO2 and Li0.5CoO2–CDP-electrolyte, and Li x C6 and Li x C6–CDP-electrolyte are examined using a C80 micro-calorimeter. For the LiCoO2/CDP-electrolyte/Li cells, the onset temperature of single Li0.5CoO2 is put off and the heat generation is decreased greatly except the one corresponding to 5% CDP-containing electrolyte. When Li0.5CoO2 coexists with CDP-electrolyte, the thermal stability is enhanced. CDP improves the thermal stability of lithiated graphite anode effectively and the addition of 5% CDP inhibits the decomposition of solid electrolyte interphase (SEI) films significantly. The electrochemical tests on LiCoO2/CDP-electrolyte/Li and Li/CDP-electrolyte/C cells show that when less than 15% CDP is added to the electrolyte, the electrochemical performances are not worsen too much. Therefore, the addition of 5–15% CDP to the electrolyte almost does not worsen the electrochemical performance of LiCoO2 cathode and graphite anode, and improves theirs thermal stability significantly; thus, it is a possible choice for electrolyte additive.