The LiNi1/3Co1/3Mn1/3O2/Ag composite used for cathode material of lithium ion battery was prepared by thermal decomposition of AgNO3 added to commercial LiNi1/3Co1/3Mn1/3O2 powders to improve the electrochemical performance of LiNi1/3Mn1/3Co1/3O2. Structure and morphology analysis showed that Ag particles were dispersed on the surface of LiNi1/3Co1/3Mn1/3O2 instead of entering the crystal structure. The results of charge–discharge tests showed that Ag additive could improve the cycle performance and high-rate discharge capability of LiNi1/3Mn1/3Co1/3O2. Extended analysis indicated that Ag was unstable in the commercial electrolyte at high potential. The improved electrochemical performance caused by Ag additive was associated not only with the enhancement of electrical conductivity of the material and the lower polarization of the cell, but also with the increased “c” parameter of LiNi1/3Mn1/3Co1/3O2 after repeated charge/discharge cycles and the compact and protective SEI layer formed on the surface of LiNi1/3Mn1/3Co1/3O2.