  Experiments on the violation of equivalence principle (EP) and solar system
give a number of constraints in which any modified gravity model must satisfy
them. We study these constraints on a kind of $f(R)$ gravity as $f(R) = R(1\pm
\epsilon \ln({R \over R_c}))$. For this investigation we use of chameleon
mechanism and show that a spherically body has thin-shell in this model. So
that we obtain an effective coupling of the fifth force which is suppressed
through a chameleon mechanism. Also, we obtain $\gamma_{PPN} = 1 \pm 1.13
\times10^{-5}$ which is agreement with experiment results. At last, we show
that for $R_c \thickapprox \rho_c$ this model is consistent with EP, thin shell
condition and fifth force of chameleon mechanism for $\epsilon \backsimeq
10^{-14}$.
