Since the discovery of carbon nanotubes (CNTs), their unique physical, chemical, mechanical and electronic characteristics have attracted much attention. Most application involving incorporation of bulk CNTs require specific characteristic in order to enhance the material properties. Therefore, there is a need to study the effect of the growth parameter on the characteristics of CNTs produced. This paper presents the study of the effect of catalyst composition on the characteristics of CNTs, namely, the diameter distribution, type of CNTs and crystallinity. Two different composition of catalysts, Mo/Fe/Al2O3 (catalyst 1) and Fe/Al 2O3 (catalyst 2) are deposited onto the substrate prior to the growth of CNTs in Chemical Vapor Deposition (CVD) system at 900°C for 10 minutes. The morphologies and the microstructure of CNTs were examined using field emission scanning electron microscope (FESEM) and Raman spectroscope. CNTs grown for these two different compositions of catalyst are found to be of different types, extreme variation of diameter size and distribution and slight difference in crystallinity. © 2010 American Institute of Physics.