Abstarct: To conduct this study we used an unbalanced panel of Iranian consumable firms over the period from 2001 to 2013. We subdivide the sample into small and large companies. The purpose of this paper is to use Dynamic Panel Data (DPD) models and GMM-system to estimate a dynamic panel data model of firm growth to see whether is Gibrat’s law holds or not. The main implication of our findings is that Gibrat's law rejected because there are some determinants which exert influence on firm growth. We find that there is a positive relationship between firm size and sales growth in both of groups.