Cobalt incorporated SBA-15 (Co-SBA-15) catalysts were prepared by using hydrothermal synthesis and over which very straight carbon microfibers were made by cracking CH4. The influence of Co loading (from 0.5 to 3 wt%) in SBA-15 and cracking temperature on the methane conversion, structures and morphologies of the microfibers was investigated. The highest yield of microfibers was obtained at 800 degrees C over the 2 wt% Co loaded SBA-15 catalyst. After 16 h reaction, the carbon fibers almost stopped further growth. The diameter of the carbon microfibers could be roughly controlled by judiciously adjusting the Co concentration in SBA-15. The carbon microfiber's growth followed a root-growth mechanism. XRD, HRTEM and Raman studies confirmed that the microfibers were graphitic. The microfiber grown over the 0.5 wt% cobalt loaded SBA-15 catalyst had a higher degree of graphitic structural order than that obtained over the 2 wt% cobalt loaded one.