The magnetic nature of lithium insertion materials of LiNi1−x Co x O2 (x =0, 1/4, 1/2, 3/4, and 1) were investigated by means of positive muon-spin rotation/relaxation (μ+SR) spectroscopy combined with X-ray diffraction (XRD) analyses and susceptibility measurements. Zero field μ+SR spectra for all the samples below 300K were well fitted by a dynamic Kubo–Toyabe function, indicating the existence of randomly oriented magnetic moments even at 2K, i.e., disordered state. The field distribution width Δ due to magnetic Ni3+ ions decreases exponentially with increasing x, suggesting that the Co substitution is likely to simply dilute Ni moments. This also supports that cobalt and nickel ions are homogeneously distributed in a solid matrix even in a muon-scale (microscopically), which is consistent with the results of macroscopic measurements.