The production procedure of ZnS:Ag:Cl blue phosphor powder is still complicated, although it is widely used for color displays. The complication mainly comes from handling of the by-product Na2S4, which covers the surface of grown ZnS particles in a crucible. The formation of Na2S4 introduces serious problems in many factors such as (i) CL intensity and color, (ii) particle size distribution, (iii) sintering of heated products, (iv) photodarkening of powders, and (v) stability of PVA phosphor slurry. These problems are clarified in this paper. It is shown that the blue phosphor powder can be produced by a more simplified procedure than before, with high reproducibility. The ZnS:Ag:Cl phosphor powder produced by the present procedure is well suitable for blue primary in color screens of FED and CRT. (c) 2005 Elsevier B.V. All rights reserved.