Mesoporous Ni-MCM-41 molecular sieves with various Si/Ni ratios 25, 50, 75, and 100 were synthesized hydrothermally, and their mesoporous structure was confirmed by various physicochemical techniques such as X-ray diffraction (XRD), N-2 adsorption studies, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The catalytic activity of these molecular sieves was tested and optimized for the maximum yield of carbon nanotubes (CNTs) in the vapor phase at various temperatures ranging from 650 to 850 C. Ni-MCM-41 (100) was found to consist of the optimum amount of metal to form large metallic clusters leading to higher carbon deposits. The optimum reaction temperature for high carbon deposits was found to be at 750 C. The maximum percentage of carbon deposit increased from 9.33 to 71.01% when the flow rate was increased from 20 to 60 mL/min. The purified CNTs were characterized by XRD, SEM, TEM, and Raman spectroscopy techniques. The analysis showed that these CNTs have larger diameters and good graphitization. The morphology of CNTs studied by TEM clearly showed the formation of carbon nanotubes with uniform diameter in the range of 25-30 nm.