The mass production of carbon nanotubes (CNTs) must be achieved for their use in real-life applications. Recently, much research has been developed to closely examine the thermal and flow fields inside reactor used for the mass production of CNTs; numerical analysis has been used as one of the methods. We have simulated the thermal and flow fields around the substrate in a thermal CVD reactor. At first, we evaluated the radiative heat transfer effect, and then we varied the growth temperature and the substrate angle. From the results of numerical analysis, and because we did not consider radiative heat transfer, the temperature distribution on the backside of the substrate was not constant. In contrast, when we considered radiative heat transfer, the temperature distribution on the backside of the substrate was constant and the calculated temperatures agree with the experiment values recorded around the substrate. Although it is more stable at a high growth temperature, the distribution of the thermal and flow fields was similar with having no concern with substrate angle and growth temperature. Therefore, a high growth temperature is a more stable conditino for the growth of CNTs.