With catalysts of cobalts supported on magnesium oxides(Co / MgO), acetylene can be pyrollyzed at 850℃. Examined by SEM, TEM and HRTEM, such thermal chemical vapor deposition products are multi-walled carbon nanotubes (MWNTs) and amorphous carbon impurities. The yields of MWNTs are varied with Co contents in the supported cata-lysts. This article proposes a more complete reaction model to illustrate growth mechanisms of MWNTs and amorphous carbons, according to crossed comparisons of thermogravimetric analysis to these products. Based on reasonable assumptions and estimations, this study con-eludes that each unit Co (Co / MgO = 1 /100) produces 0.33wt% MWNTs on total supported catalysts when Co contents are not high.