PURPOSE: To obtain phosphor particles completely coated with a polymer film by treating the surfaces of phosphor particles with a surface modifier. coating the surfaces with a polymerizable monomer, and polymerizing the monomer.

CONSTITUTION: Phosphor particles coated with a polymer are obtd. by treating the surfaces of 1 pt.wt. phosphor particles with 0.001-0.1 pt.wt. surface modifier which has, in the same molecule, both a group having the affinity with the phosphor particle and a group having the affinity with a monomer for coating the particles or its polymer to modify the surfaces, mixing 1 pt.wt. phosphor particles treated as above with 0.001-10 pts.wt. polymerizable monomer contg. 0.01-10wt.% polymn. initiator to coat the surfaces with the monomer, dispersing the particles in a dispersion medium such as water using a dispersion mill, if necessary in the presence of a dispersant such as a surfactant, polymerizing the monomer on the surfaces at room temp. to 90°C for at least 15hr, and separating, washing, and drying the particles.