Under high optical power operation, (AlGa)As DH lasers suffer from catastrophic optical damage (COD). When the COD occurs dark line defects are observed in the bulk crystal, as well as in mirror surface damage. The COD generation mechanism is clarified by TEM observation of the defects associated with the COD and by investigation on transient behaviors of lasing light excited by one-shot-pulse. It is confirmed that such defects are generated by local overheating up to the melting point of GaAs due to the thermal positive feed back and following transfer of molten zone.