The influence of Mn activation on CaSO4:Dy thermoluminescent (TL) phosphor has been observed to study the basic TL characteristics such as TL output, fading, sensitivity, and dose response. The experimental results indicate that the TL characteristics are strongly influenced by the concentration of Mn dopant in CaSO4:Dy, especially for the low temperature trapping peak of the glow curves. The TL properties of CaSO4:Dy:Mn phosphor with a concentration ratio of 1:4 of Dy to Mn include not only high sensitivity, negligible fading, and linear dose response as compared to the CaSO4:Dy phosphor, but also two well-separated peaks and relative peak heights. This suggests that it can be used for accidental dose and elapsed time evaluations as well as for a routine personnel dosimeter. It was also found that 350 degrees C to 400 degrees C for 30 min is the optimum temperature and time in annealing treatment for this phosphor.