(La0.47Gd0.2)Sr0.33MnO3 nanoparticles were synthesized by an amorphous heteronuclear complexing method. The microstructure and magnetic properties of the synthesized nanoparticles were characterized by XRD, HRTEM and MPMS. XRD and selected area electron diffraction (SAD) analyses reveal that the products are of pure single-phase rhombohedral structure. TEM observation shows that the particle sizes of the products calcined at 600, 800 and 1000 oC for 10 h are about 40-50 nm, 90-100 nm and 140-150 nm, respectively. The Curie temperature TC (298 K) of the nanoparticles does not depend on the particle size, while the relative cooling power (RCP) is determined by the particle size. (La0.47Gd0.2)Sr0.33MnO3 with particle sizes ranging from 90 nm to 100 nm has the largest RCP, which may be a suitable candidate as working substance in magnetic refrigeration at room temperature.