We fabricated and life‐tested visible AlGaAs/GaAs channeled‐substrate‐planar‐type lasers and found that some are degraded within 100 h of operation. We investigated these using photoluminescence and transmission electron microscopic images. The characteristic features in photoluminescence images are dark defects that run parallel to the channeled stripe. Using transmission electron microscopy, we found that these dark defects are composed of precipitates, dislocation loops, and dislocation dipoles. In order to know the relation between these degradation phenomena and the thermal strain induced by lattice mismatch, we calculated the stress distribution in the semiconductor laser using the finite element method. According to this simulation, a stress concentration in the active layer arises near the edges of the channeled stripe where the volume dilatation is maximal. Combining the results of experiments and simulation, we concluded that interstitial atoms created by nonradiative recombination migrate to the edges of the stripe, and that ultimately dark defects appear near the edges of the stripe.