Metamagnetism in YCo₂ and LuCo₂ is investigated in the spin-fluctuation theory, considering the contribution from both the zero-point and the thermal spin fluctuation. The metamagnetic transition at low temperature and the temperature dependence of the paramagnetic susceptibility at finite temperature are investigated in a self-consistent numerical calculation based on the spin-fluctuation theory and good agreements with experimental results have been obtained in a systematic way. It is shown that the effect of strong enhancement of the paramagnetic susceptibility and the electronic specific coefficient at low temperature can be explained satisfactorily by taking into consideration the zero-point spin fluctuation.