The present invention provides an organic/inorganic composite porous separator, which comprises: (a) a porous substrate having pores; and (b) an organic/inorganic composite layer formed by coating at least one region selected from the group consisting of a surface of the substrate and a part of pores present in the substrate with a mixture of inorganic porous particles and a binder polymer, wherein the inorganic porous particles have a plurality of macropores with a diameter of 50 nm or greater in the particle itself thereby form a pore structure, a manufacturing method thereof, and an electrochemical device using the same. As an additional pathway for lithium ions is created due to a number of pores existing in the inorganic porous particle itself, degradation in the battery performance can be minimized, and energy density per unit weight can be increased by the weight loss effect.