The goal of this paper is to develop a simulation software for the ethyl benzene production unit in a Petrochemical Complex of Iran. Ethyl benzene whose consumption has rapidly increased in recent years, is the feed for production of styrene monomer. This material is produced from alkylation of benzene with ethylene. A process simulator can play a very important role in the development of this process. In this research, alkylation of benzene for production of ethylbenzene has been simulated. First the mass, energy and momentum balance equations have been developed for the axial flow reactor. Optimization techniques have then been applied to modify the kinetic equations presented in literature to somehow satisfy the unit conditions. Finally the model predicted values such as pressure, reactor temperature, reactant conversion and products compositions have been compared with those of experimental ones adopted from the industrial unit. The comparison reveals that the model predicted values are adequately in compliance with the experimental data and hence can be used, with sufficient accuracy, to design pilot plants and other new units.