A method of preparing core-shell submicron spheres is disclosed. The method comprises steps hereafter. An emulsion containing a core monomer is heated to a temperature of 10° C. below its boiling point to the boiling point. A core forming step is performed by adding an initiator solution to the heated emulsion to form cores by polymerizing the core monomer. When the conversion of the above polymerization is 10% to 95%, a shell forming step is performed by adding a shell monomer into the emulsion, at a temperature of 50° C. below the emulsion's boiling point to the boiling point thereof, to form shells, respectively surrounding the cores, by polymerizing the shell monomer.