$M_\pi\approx 300$ MeV. Carrying out NLO analyses in SU(2) and SU(3) chiral perturbation theory, this work found $\langle r^2\rangle$ to be consistent with the experimental value at the upper edge of a 10% error band.

In this paper we present our calculation of the electromagnetic form factor of pion in $N_f=2+1$ dynamical flavor QCD using the Wilson-clover quark action. For measurements we employ the $N_f=2+1$ PACS-CS gauge configurations generated on a $32^3\times 64$ lattice using the Iwasaki gauge action and the Wilson-clover action at a lattice spacing estimated to be $a=0.0907(13)$ fm at the physical point [@pacscs]. Since the pion mass on this gauge configuration set covers the range from $M_\pi\approx 700$MeV down to $156$ MeV, we are able to examine both the known range above $M_\pi\approx 300$MeV and a novel range below toward the physical pion mass.

The paper is organized as follows. In Sec. 2 we present our method to calculate the pion form factor. In order to access the region of small momentum transfer, we use the method of partially twisted boundary condition [@boyle2007; @sachrajda2005; @bedaque2005; @jian], and in order to fight increasing computational cost for smaller pion mass, we apply the method of random wall source 