Diallyl phthalate resin plates showed favorable characteristics as fission track detectors. The detection efficiency of fission fragments was about 100% for both perpendicular and random incidence. DAP plates were insensitive to alpha particles and fast neutrons. The plates possessing fission tracks were etched after heat treatment or after gamma-ray irradiation. Heat treatment above 160°C lasting for 1 hour and irradiation above 10kGy increased the etch-pit diameter. A reduction of the track detection efficiency was observed in the plates exposed to heat treatment of 260°C lasting for 1 hour and irradiations of 2MGy and above. These phenomena were observed in the study of plates in which fission tracks were made after the above- mentioned heat treatment or irradiation. These results suggest not only the conditions under which the plates may be used, but also the possibility of estimation of a detector's thermal history or gamma-ray dose.