We report on effects of thermal conductivities of substrates on self-diffraction rings from guest-host liquid crystals (GHLCs). A He-Ne laser-induced self-phase modulation due to photothermal effects in the GHLC. A far-field pattern of the laser beam passed through the GHLC cells depended on the thermal conductivities of the substrates of the GHLC cells, and the results were well explained by three-dimensional heat-conduction analysis and Kirchhoff's diffraction theory.