A green emitting SrAl2O4:Eu2+,Dy3+ phosphor with very bright and long lasting phosphorescence has been newly developed. The incorporation of Dy3+ ion into the SrAl3O4:Eu3+ system as an auxiliary activator dominates the phosphorescence, thermoluminescence, and photoconductivity characteristics of the phosphor to a large extent. Evidence is presented for the mechanism that the phosphorescence is ascribed to the photoconductivity due to holes, and to the trapping and thermal release of tile holes by Dy3+ ions in the system. The incorporation of the Dy3+ ion forms a highly dense trapping level located at a suitable depth in relation to the thermal release rate at room temperature, thus producing the very bright and long phosphorescence.