Disclosed is an organic/inorganic composite separator including: a porous substrate having pores; and a porous active layer containing a mixture of inorganic particles and a binder polymer with which at least one surface of the porous substrate is coated. The organic/inorganic composite separator of the present invention may be useful to enhance peeling and scratch resistances and improve a lamination characteristic by introducing a porous active layer onto a porous substrate having pores, the porous active layer having heterogeneity of morphology toward a thickness direction in which a content ratio of the binder polymer/inorganic particles present in a surface layer is higher than that of the binder polymer/inorganic particles present inside the surface layer. Accordingly, the stability and performances of a battery can be improved together since the detachment of inorganic particles from the porous active layer may be reduced during the assembly process of the electrochemical device.