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 $xsim$ 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.
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