In this study, we investigated the role of Mo on the growth of single-walled carbon nanotubes (SWNTs) in arc discharge method. Mo with various concentrations was incorporated into two typical arc systems, namely Ni/Y-He and Fe-Ar/H2. Based on analyses of yield and purity, we found the best concentration of Mo is 0.5 at% in Ni/Y-He system and 0.3 at% in Fe-Ar/H2 system. For both systems, the incorporation of Mo has dramatically increased the yield of soot in their best concentrations. On the contrary, the promotion effects of Mo on purity of SWNTs were not same. Thermal gravimetric analysis showed that with the addition of Mo, the purity of SWNTs did not change effectively for Ni-Y/Mo-He system, but noticeable increment of purity was observed for Fe/Mo-Ar/H2 system from 33 wt% to 50 wt%. Considering on these experimental results, we prove that Mo plays a positive role in arc discharge for the enhancement of SWNTs growth.