  We present preliminary results on the $\rho$ meson decay width from $N_f=2+1$
full QCD configurations generated by PACS-CS Collaboration. The decay width is
estimated from the $P$-wave scattering phase shift for the isospin $I=1$
two-pion system. The finite size formula presented by L\"uscher in the center
of mass frame and its extension to non-zero total momentum frame by Rummukainen
and Gottlieb are employed for the calculations of the phase shift. Our
calculations are carried out at $m_\pi=410\ {\rm MeV}$ ($m_\pi/m_\rho=0.46$)
and $a=0.091\ {\rm fm}$ on a $32^3\times 64$ ($La=2.9 {\rm fm}$) lattice.
