  We investigate the modified gravity theories in terms of the effective dark
energy models. We compare the cosmic expansion history and the linear growth in
different models. We also study the evolution of linear cosmological
perturbations in modified theories of gravity assuming the Palatini formalism.
We find the stability of the superhorizon metric evolution depends on models.
We also study the matter density fluctuation in the general gauge and show the
differential equations in super and sub-horizon scales.
