The determinants of vertical integration was studied by a simple model.The model was an extension of GH model.It was found that the relationship between a downstream(producer) industry and an upstream(supplier) industry was likely to be backward vertically integrated when the producing industry was more productive and the supplying industry was less productive,and vice versa.The integration was accelerated when the two enterprises relied on each other deeply and was controlled by the high competitivity of the cross-market.The results were popular and consistent with the incomplete contract theories.