This paper reports new observations in degraded (Al · Ga)As double‐heterostructure (DH) laser diodes and proposes a mechanism of short‐term ([inverted lazy s] 100 h) degradation characteristic to DH structures. Degradation is seen localized as ``dark lines'' of certain crystalline orientations in electroluminescent patterns, as well as junction current patterns using an SEM. X‐ray measurement showed internal strain due to heteromismatch. It is suggested that the internal stress accelerates development of the dark lines, which is likely to be a crystalline defect caused by thermal stresses.