Effects of the composition on the afterglow characteristics of SrAl2O4:Eu2+, Dy3+ phosphors have been evaluated using crystals grown by a floating zone technique for the first time. Phosphorescence spectrum due to Eu2+ ions peaking at lambda = 520 nm does not vary with the compositions. Persistent times vary greatly with different compositions. Dy concentrations added as auxiliary activators are changed dramatically with the Sr/Al molar ratio of the starting materials. The phosphorescence decay consists of three exponential components with different decay times. The faster decay components play an important role for the long persistent phosphorescence.