The magnetic properties of Li[Li (1/3− x/3) Mn (2/3−2 x/3) Ni x ]O 2 ( x = 0.4) are investigated by dc magnetization measurements. The high-temperature paramagnetic susceptibility can be fitted by Curie–Weiss law whose Curie and Weiss constants are 0.95(2) emu K/mol Oe and −70(4) K, respectively. The ZFC/FC curves of Li 0.2Ni 0.4Mn 0.4O 2 show a strong irreversibility behavior and T irr shifts to lower temperature with the increase of applied magnetic field. Together with de Almeida–Thouless (AT) line analysis, spin-glass-like state is suggested to be the ground state of Li 0.2Ni 0.4Mn 0.4O 2. In addition, the frustration parameter θ / T f is calculated to be about 7.8, lower than the value that frustration effect is strong enough to give rise to spin-glass behavior. It is concluded that the spin-glass-like behavior results from the short-range structure disorder rather than the geometrical frustration.