Rapid degradation has been observed for InGaAsP/InP double heterostructure lasers. It has been found that the rapid degradation is due to ⟨110⟩ dark line defects generated parallel to the stripe and its occurrence depends on the thickness of the InGaAsP contact layer. It has been shown that formation of a InGaAsP contact layer of more than 0.5‐&mgr;m thickness is necessary in order to realize high reliability of the lasers.