In this study, liquid furnace has been investigated to find out the reason of over-cracking which results in excessive tar and coke formation so that the furnace run length is decreased. For this purpose, a one-dimensional mathematical model is used to analyze the furnace temperature profile. In addition, inlet feedstocks and outlet gas products of furnace have been studied. The results obtained from the simulation demonstrate differences between modeled coil outlet temperature and cracking severity with those of experimental data. For overcoming this problem, furnace temperature were sufficiently reduced with the purpose of reaching constant cracking severity at 0.5. Correction of the coil outlet temperature led to significant increase in cracking severity, propylene production, furnace capacity and run length while gasoline production and number of decoke cycles decreased. (C) 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.