A separator, an electrode assembly and an electrochemical device including the same are provided. The separator is used for a zigzag stacking-type electrode assembly, and includes a porous substrate made of a polymer material and a porous coating layer formed on opposite surfaces of the porous substrate and containing inorganic particles. A thickness (Ts) of the porous substate decreases gradually from opposite ends toward a central portion based on a longitudinal direction of the separator, a total thickness (Tc) of the porous coating layers increases gradually toward the central portion, and a total thickness (Ts + Tc) of the separator is constant.