We have found that nanoparticles of silver iodide have high sensitivity for blue light and are fixed during thermal development in photothermographic materials. Silver iodide particles exhibit high sensitivity and weak LIRF in photothermographic materials, as they do in conventional materials with higher silver ion concentration than usual, and have their sensitivity further improved by reducing agents adsorptive to the particles. The sensitivity of photothermographic materials with nanoparticles of silver iodide for blue laser light could be comparable to that of the materials with silver iodobromide for red laser light. Silver iodide particles are slowly fixed during thermal development in photothermographic materials with existence of either silver carboxylate or phthalazine, and rapidly fixed with coexistence of these two compounds. Fog density of developed layers of photothermographic materials with nanoparticles of silver iodide does not increase under room light illumination.