Finally, all data were subjected through a statistical analysis. From this study we have seen that the majority of the samples had Log-Normal diameter distributions by adjusting oxidizer amount and pretreatment timing. We were able to control CNT diameters within very narrow diameter ranges.This thesis work focuses on growing high quality carbon nanotubes using ethylene as hydrocarbon gas in thermal chemical vapor deposition method in presence of weak oxidizers. We carried out experiments to study the ratio of amount of CO2, O2 and H2O in pretreatment to growth and the timing of the metal-oxide to metal conversion in presence of the oxidizers.Firstly, the effects of CO2 addition were investigated on a variety of growth conditions to study the controlled diameter growth of CNTs. The main growth parameters controlled in this work in the CNT growth were CO2 level, pretreatment time and temperature. Pretreatment times of 15, 10, 5, 2 minutes were studied. CO2 flow rates were send in system during both pretreatment time and growth time at 10:1, 10:2, 5:1, 5:2, 2:1 and 2:2 sccm ratios, respectively. We also studied two different growth temperatures at 740 oC and 760 oC.Secondly, the influence of O2 for effective CNT growth was investigated by utilizing different pretreatment times and O2 levels. Pretreatment times of 15, 10, 5, 2 minutes were studied. O2 gas flow rates during pretreatment time and growth time were studied as 5:1, 5:2, 2:2, 2:1, 2:0.5, respectively. As a result, we obtained small diameters with the pretreatment time of 15 min. in the presence of O2.Thirdly, the effects of H2O vapor for effective CNT growth on Fe/Al2O3/SiO2 were examined. During the experiments, pretreatment time was kept at a constant value of 15 min. and H2O vapor content in Ar was varied. The results of examined analyses show that the best results were obtained at the growth 60 o C (A) / 60 o C (A).