The tough market situation for ethylene production; has accelerated the development of a more rigorous and reliable cracking model. However, thermal cracking of naphtha has such numerous reaction routes and intermediate radicals and molecules that the detailed reaction mechanism has not yet been determined. This research is aimed at developing a rigorous but practical reaction mechanism for an industrial cracker model. First, the reaction mechanism set for naphtha cracking is generated on the basis of major reaction classes in pyrolysis and feed components. To reduce the computational load, the reaction mechanism set is reduced using the eigenvalue-eigenvector decomposition method. To compensate for the uncertainty in the kinetic parameters for-a plant, the mechanism is customized based on the results of a sensitivity analysis. The constructed reaction mechanism can be used to optimize the operating conditions of the naphtha cracker by precise estimation of the production yield of a given naphtha sample with a manageable computational load and flexibility toward industrial practices.