In this thesis unsteady mathematical model include hydrodynamic effects and the equation of mass and energy transfer in a trickle bed reactor was developed to investigate the behavior of Tehran Refinery isomerization reactor. The mechanism used for the kinetics of the reactions to the isomerization reactor contains 25 response were considered. The model’s equations were solved with MATLAB software and results in order to verify the error was reported. Most of the reported error for the predicted cycloparaffins output of the unit is equal to 15.9 percent. The developed model able to predict output characteristics of unit, including octane number of output products, paraffin’s isomerization number, x factor, normal pentane and normal hexane. The characteristics of the inlet temperature, the ratio of hydrogen to hydrocarbons, liquid space velocity and pressure on the isomerization reactor performance was evaluated and it was observed that increasing the temperature from 363 to 408 K, by changing the ratio of hydrogen to hydrocarbon of 0.2 to 1 and by increasing liquid space velocity of 1 to 6 per hours octane number were 1, 4 and 6 units is reduced. Keywords : Modeling, Simulation, Isomerization, Light Naphtha, Tricle Bed Reactor