We have investigated effects of sulfur addition as a promoter on the growth of carbon nanotube (CNT) and attempted to provide insights into the role of sulfur during CNT formation via catalyst characterization. CNTs were synthesized on a quartz substrate using sulfur in addition to conventional catalysts, cobalt-molybdenum co-catalyst, through chemical vapor deposition employing carbon monoxide as a carbon source. Sulfur powder was used as a sulfur source in this work. Some characterization approaches such as atomic force microscope, high-resolution transmission electron microscopy, time-of-flight secondary ion mass spectroscopy, X-ray photo- luminescence spectroscopy and chemical equilibrium calculation were performed in order to investigate the role of sulfur in CNT synthesis. It was suggested that sulfur and catalytic metal formed sulfides particularly in the vicinity of surface, resulting in the acceleration of CNT growth probably due to the enhancement of atomic carbon diffusion.