The relationships between microwave photoconductivity, photographic sensitivity, and silver halide crystal size in nanometer-size silver halide ultrafine crystals employed in photothermographic dry imaging materials were examined. The behavior of photoelectrons produced by the light irradiation of nanometer-size silver halide crystals indicated that photoconductivity depends on crystal size and, further, that this dependency differs from that found in the micron-size silver halide crystals used in conventional photographic materials. Furthermore, while photographic sensitivity depends on the particle size of silver halide crystals in both photothermographic dry imaging materials and in conventional photographic materials, that dependency was found to be lesser in photothermographic dry imaging materials.