NOVELTY - Preparing core nanoparticles supported on a support for a core-shell structured electrocatalyst, comprises reacting a support with a precursor of at least one core-forming metal in an ether-based solvent.    USE - The method is useful for preparing core nanoparticles supported on a support for a core-shell structured electrocatalyst that is used in an electrode, a cathode and an anode for a fuel cell (all claimed).    ADVANTAGE - The method: does not require a post-treatment process (chemical and heat treatments); provides core-shell structured electrocatalyst with a large amount of supported catalyst, and superior catalytic activity and excellent electrochemical properties, in which a shell layer is selectively and uniformly formed only on the surface of nanosized core particles having a uniform particle size supported on a support; and can maintain the form of core particles supported on a support unchanged.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) producing a core-shell structured electrocatalyst for a fuel cell, comprising reacting the support with the precursor of core-forming metal in the ether-based solvent to obtain core nanoparticles supported on the support, and reducing a precursor of at least one shell-forming metal using an ester-based reducing agent in a solution in which the core nanoparticles supported on the support are dipped or dispersed;    (2) a core-shell structured electrocatalyst for a fuel cell, comprising a support, core nanoparticles supported on the support, and a shell layer selectively formed on the surface of the core nanoparticles, where the core is composed of at least one metal or its alloy comprising platinum, palladium, ruthenium, rhodium, osmium, transition metals and/or their alloys, and the shell layer consists of at least one layer, each composed of at least one metal or its alloy comprising platinum, palladium, iridium, ruthenium, rhodium, osmium, transition metals and/or their alloys;    (3) an electrode, a cathode or an anode for a fuel cell, comprising the core-shell structured electrocatalyst;    (4) a core-shell structured electrocatalyst (A) for a fuel cell, comprising a support and many core-shell structured catalysts supported on the support, where the cores are composed of palladium and the shells are composed of an alloy of palladium and iridium; and    (5) an anode for a fuel cell, comprising the core-shell structured electrocatalyst (A).