Bed material size frequency distribution of rivers is one of the key points to identify river characteristics. River bed armoring development is one of the controlling factors in sediment transporting and eroding processes of gravel bed rivers. In this research, field study was initially carried out on several gravel bed rivers in Khuzestan and Chaharmahal provinces. 24 hydrometeorological stations in 17 river reaches were sampled to characterize bed material gradation curve. With regards to bed structure of gravel bed rivers, samplings were made by surface and volumetric methods. Thereafter, sediment samples were differentiated in three categories of surface, armor and sub-armor layers. The collected field data were originally applied to derive regime relationships by using multivariate regression analysis. The effect of bed structure was directly studied in these relationships for the better understanding of that in the regime relationships.