The effect of massive neutrinos on matter power spectrum is discussed in the context of f ( R ) gravity .
It is shown that the anomalous growth of density fluctuations on small scales due to the scalaron force can be compensated by free streaming of neutrinos .
As a result , models which predict observable deviation of the equation-of-state parameter w _ { DE } from w _ { DE } = -1 can be reconciled with observations of matter clustering if the total neutrino mass is O ( 0.5 ~ { } { eV } ) .