Some mixtures of two active materials are utilized for the blended cathodes in lithium-ion batteries. The blended cathode and unitary cathodes were prepared by a layer compound, LiNi1/3Mn1/3Co1/3O2, and a spinel compound, LiAl0.1Mn1.9O4 as simple active materials in order to develop the method to estimate the capacities of the both active materials in the blended cathode. The differential curves (dQ/dV) clearly provided the information on the redox voltage region of the layer and the spinel compounds in the unitary and blended cathodes. A regression curve of the layer compound was partially calculated using the dQ/dV of blended cathode in the layered redox voltage region, and it was extrapolated until the mixed redox voltage region of the layer and spinel compounds. The capacity in the blended cathode was divided into the capacities of the active materials by this curve. This method was also successfully applied to the degraded blended cathode. This method proposed in this paper is a significant technique for degradation analysis of both active materials in the blended cathode without the need for the same active material lots as those of blended cathodes, using information on electrochemically active voltage ranges of each active material such as references.