As the oxidation rate of the fuel clad is an important factor in fuel designation, so its calculation for Ex-reactor and In-reactor conditions is of high importance. Since, the Zirconium oxidation is of high importance, so numerous studies are accomplished in order to study the layer and various methods have been proposed for reducing its growth. While, many models are developed to calculate the thickness of this layer both for external and operation condition of the reactor. These models are used for calculations, designation and studying the function of fuel rod. The corrosion of fuel clad is defined in two ways, initially the corrosion is calculated regardless of reactor's impact, so called Ex-reactor, then it is computed by taking into account the effects, including irradiation, Lithium concentration, etc. which is known as In-reactor. The present paper aims at calculating the corrosion rate of fuel rod clad for Bushehr Reactor in Ex-reactor, and finally the reactor will be proposed by considering the effects.