Chemical Vapor Deposition (CVD) is one of the most important methods for producing Carbon Nanotubes (CNTs). In this research, a numerical model, based on finite volume method, is investigated. The applied method solves the conservation of mass, momentum, energy and species transport equations with aid of ideal gas law. Using this model, the growth rate and thickness uniformity of produced CNTs, in a horizontal CVD reactor, at atmospheric pressure, are calculated. The furnace temperature and inlet hydrocarbon concentration variations are studied as the effective parameters on CNT growth rate and thickness uniformity. It is indicated that by increasing the furnace temperature, the CNT growth rate increases, while the thickness uniformity shows decreasing. The results show that the growth rate of produced CNTs could be improved by increasing the inlet hydrocarbon concentration, but the latter causes more non uniformity on the CNTs height.