We present our preliminary result of the electromagnetic form factor of the pion on the large volume configurations $L = 8.1$ fm with $m_\pi = 0.145$ GeV and $m_K = 0.525$ GeV using the stout-smearing clover quark and Iwasaki gauge action at $a^{-1}=2.333$ GeV. From the results at small momentum transfer we determine the mean square charge radius at the physical $m_\pi$ and $m_K$ using the next-to-leading order formula of the SU(2) chiral perturbation theory with the strange quark mass reweighting technique. We estimate the systematic error coming from the strange quark mass extrapolation to the physical value. We compare our result of the charge radius with those of the experiment and other lattice calculations.