A method for forming a very fine phosphor screen of a monochrome cathode ray tube whose roughness is inconspicuous even if the picture is enlarged. White phosphors (P45) whose average particle diameter D by a dry method is 1.5 .mu.m, ethanol, and ammonia water are stirred and maintained at 50.degree. to 55.degree. C. A mixed solution of ethyl silicate and ethanol as well as ammonia water are separately dropped into the mixed solution containing the phosphor particles (P45). After standing still, the supernatant liquid is removed by decantation, and the precipitates are dehydrated, and dried by heating, and sieved to obtain white phosphors (P45) whose surfaces are coated with silicon dioxide. Such phosphors satisfy 1D.ltoreq.d.ltoreq.1.3D, in which "D" represents an average particle diameter measured by a dry method, and "d" represents an average particle diameter measured by a wet method. Therefore, the phosphors have excellent dispersion property in a suspension.
