This paper reports the effect of a binary conductive agent consisting of two kinds of carbon particles with different sizes in a LiCoO2 cathode on the performance of a lithium ion polymer battery. Super-P (30 nm) and Lonza-KS6 (6 μm) were selected as the components for the binary conductive agents. The total amount and the ratio of the two components of the binary conductive agents were varied. At a fixed Lonza/Super-P ratio of 1:1, the cathode with the higher conductive agent content showed a lower electrical resistance and a lower density, which resulted in better rate and cycling characteristics. However, this advantage was somewhat offset by the decrease of energy density. At a fixed binary conductive agent content of 8 wt.%, the electrode density increased and the surface electrode resistance decreased with increasing Super-P content. In spite of the decreased ionic diffusion rate, the charge transfer resistance was decreased and the rate capability was increased with increasing Super-P content, which indicated that the formation of effective electronic conduction paths was highly important. The SEM microscopic observation and measurement of planar density of LiCoO2 indicate that the addition of a small amount of Lonza-KS6 provides a uniform dispersion of LiCoO2 particles in the cathode. As a consequence, the cell with the binary conductive agent at an appropriate Lonza/Super-P ratio had a better cycle life than those with single conductive agents.