In the present study, a mathematical model for naphtha thermal cracking in the presence of steam is developed. This model can predict the behavior of cracking unit of the olefin plant. In this plant a mixture of steam and naphtha passes through cracking tubes. The major role of steam is partial removal of coke that causes different problems along the reactor such as low heat transfer and high pressure drop. In this study another diluting media such as carbon dioxide is employed instead of steam in order to investigate the effect of carbon dioxide on main products yield and operating conditions. Carbon dioxide was shown to improve not only naphtha conversion but also the yield of ethylene and other main products. It also increases the heat transfer to the coils due to the fact that in presence of carbon dioxide the coke thickness along the reactor reduces dramatically. This analysis suggests that the run length of the reactor in presence of carbon dioxide is 25% higher than that in presence of steam. It is also observed that using carbon dioxide reduces tube wall temperature, resulting in less operating and maintenance costs. (C) 2013 Elsevier B.V. All rights reserved.