© 2019, Japan Laser Processing Society. Photoelastic observation was applied for the thermal stress cleaving of brittle substrates in order to experimentally investigate the stress distribution and its effects on the accuracy of crack propaga-tion. In the laser cleaving process with a curved path, the principle stress acted on the incorrect direc-tion near the edge of the substrate, then the crack deviated from the desired cleaving path. Additionally, high scanning speed of laser disturbed the distribution of the principal stress direction and resulted in the incline of cleaved surface. In the trimming process by laser cleaving, the straight cleaving path was bent toward the free edge of substrate due to the asymmetrical stress distribution, which was successfully observed by the proposed photoelastic method.