TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: In the method, the first mixture solution further comprises a gadolinium precursor. The thermal treatment performed during the nanoparticles formation step is carried out at temperatures of 200-370 degrees C for 30 minutes to four hours. The nanophosphor preparation method further involves: cooling the thermally treated nanoparticles, and cleaning the cooled nanoparticles with acetone or ethanol, subsequent to the nanoparticles formation step. The method further involves: a shell formation step subsequent to the nanoparticles formation step, and where the shell formation step involves: a shell solution preparation step for preparing a third mixture solution comprising sodium oleate and lanthanide precursor including gadolinium precursor; a nanoparticles mixing step for performing a thermal treatment on the third mixture solution to form gadolinium oleate, dissolving the gadolinium oleate in a solution containing oleic acid and 1-octadecene, and mixing nanoparticles that have been formed during the nanoparticles formation step with the solution to prepare a fourth mixture solution; a shell reaction solution preparation step for mixing a solution containing a sodium precursor, a fluorine precursor and alcohol with the fourth mixture solution to prepare a shell reaction solution; and a shell formation step for removing alcohol from the shell reaction solution and performing a thermal treatment on the shell reaction solution from which alcohol is removed to form a shell on a surface of the core containing the nanoparticles. Preferred Components: The nanophosphor comprises a core containing the nanoparticles and a shell located on a surface of the core, and the shell comprises NaGd1-vMvF4 (II). The lanthanide element is any one selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Tm and Lu. The size of nanoparticles is 1-10 nm. The nanoparticle has a hexagonal structure. The size of nanophosphor is greater than 1 nm and less than or equal to 20 nm. The nanophosphor has up-conversion properties and magnetic properties. The yttrium precursor is any one selected from yttrium acetate (Y(CH3COO)3), yttrium chloride (YCl3), and yttrium chloride hexahydrate (YCl3.6H2O); the ytterbium precursor is any one selected from ytterbium acetate (Yb(CH3COO)3), ytterbium chloride (YbCl3), and ytterbium chloride hexahydrate (YbCl3.6H2O); and the erbium precursor is any one selected from erbium acetate (Er(CH3COO)3), erbium chloride (ErCl3), and erbium chloride hexahydrate (ErCl3.6H2O). The gadolinium precursor is any one selected from gadolinium acetate (Gd(CH3COO)3), gadolinium chloride (GdCl3), and gadolinium chloride hexahydrate (GdCl3.6H2O).v=0.1-1;M=Y' or lanthanide elements. EXAMPLE - Yttrium chloride hexahydrate (YCl3.6H2O) (0.8 mmol), ytterbium chloride hexahydrate (YbCl3.6H2O) (0.18 mmol), and erbium chloride hexahydrate (ErCl3.6H2O) (0.02 mmol) were mixed with the solvent comprising oleic acid (6 ml) and 1-octadecen (15 ml) and a first mixture solution was prepared. The lanthanide compound was dissolved in the solvent by heating the first mixture solution at 150 degrees C to form a clear solution, and thus the solution containing a lanthanide complex was formed. A second mixture solution in which sodium hydroxide (2.5 mmol) and ammonium fluoride (4 mmol) were mixed with methanol was mixed with the solution containing the lanthanide complex, and then mixed to prepare a reaction solution. Methanol was removed from the reaction solution, and then the reaction solution from which the methanol was removed was thermally treated at 300 degrees C for 90 minutes under argon atmosphere. While performing the thermal treatment, a nanophosphor of beta -NaY0.8F4:Yb3+0.18,Er3+0.02 having a hexagonal p-phase NaYF4 structure and size of 21.5 nm was prepared.