The present disclosure is related to a method to control sizes of core-shell nanoparticles comprising the steps of: manufacturing slurry by irradiating ultrasonic waves to a dispersion solution containing a reducing solvent, a carbon support, a transition metal precursor and a precious metal precursor; manufacturing a solid by filtering the manufactured slurry, followed by washing and drying; and manufacturing a nanoparticle of a transition metal core and a platinum shell by heat-treating the dried solid at a temperature of 450 to 900° C. and a pressure of 1 to 90 bar for 0.5 to 10 hours under N2 atmosphere.