We investigated effects of delithiation on crystal, electronic structure, thermodynamic stability and physical property of Li x Ni0.5Mn0.5O2 by using a chemical delithiation treatment with ammonium peroxodisulfate, and compared the results with those of electrochemical delithiation. X-ray diffraction and thermodynamic data indicated that there was no phase transition in the samples after the chemical delithiation treatment within the lithium range of 0.50≤ x ≤1.00. From the ICP measurements and potentiometric titration, it was found that the chemical treatment successfully remove Li from the structure without any elution of Ni and Mn, but it accompanied O2 removal at an initial process of the delithiation, i.e. 0.80≤ x ≤1.00. Crystal and electronic structure analyses also suggested such a O2 removal at the lithium content range in the chemical delithiation method, although the estimated structure changes below x =0.80 showed similar tendencies to those by the electrochemical delithiation.