  We investigate superconductivity and transport properties of Co doped
SmFe$_{1-x}$Co$_{x}$AsO system. The antiferromagnetic (AFM) spin-density wave
(SDW) order is rapidly suppressed by Co doping, and superconductivity emerges
as $x$ $\geq$ 0.05. $T_c$$^{mid}$ increases with increasing Co content, shows a
maximum of 17.2 K at the optimally doping of $x\sim$ 0.10. A phase diagram is
derived based on the transport measurements and a dome-like $T_c$ versus $x$
curve is established. Meanwhile we found that the normal state thermopower
might consist of two different contributions. One contribution increases
gradually with increasing $x$, and the other contribution is abnormally
enhanced in the superconducting window 0.05 $\leq$ $x$ $\leq$ 0.20, and shows a
dome-like doping dependence. A close correlation between $T_{c}$ and the
abnormally enhanced term of thermopower is proposed.
