It is an object to provide a cathode active material and a cathode which can attain a lithium ion secondary battery with high capacity and high security, and further to provide the lithium ion secondary battery with high capacity and high security.According to the present invention, the cathode active material is represented by the following composition formula: Li1.1+xNiaM1bM2cO2 wherein M1 represents Co, or Co and Mn; M2 represents Mo, W or Nb; −0.07≦x≦0.1; 0.6≦a≦0.9; 0.05≦b≦0.38; and 0.02≦c≦0.06.