The invention relates to a cathode active material and a lithium ion secondary battery comprising the same, and provides the cathode active mateiral and the lithium ion secondary battery with high durability. The cathode active mateiral is represented as the general formula of Li(1+a)NixCoyMnzWtO2 (-0,05 <= a <= 0,2, x = 1 - y - z - t, 0 <= y < 1, 0 <= z < 1, 0 <= t <= 0.03). The cathode active material is characterized in that: average element concentration inside primary particles and at grain-boundary of the primary particles of the element W is set as t1, and the standard deviation of the element concentration inside primary particles and at grain-boundary of the primary particles of the element W is set as [sigma]1, the following equation is satisfied: [sigma]1/t1 <= 0.92.