Abstract: Based on the study of the metallographic structure of the non-quenched and tempered N80 OCTG -purpose steel,an analysis is made to identify the causes responsible for development of typical structural defects of the steel and the influence by such defects on the mechanical properties thereof. The analytical result shows that the typical structures of the domestically-used non-quenched and tempered N80 OCTG-purpose steel are P+F and B+P+F. Relevant main structural defects are course network ferrite,flake martensitic structure,overheated Widmannst?tten structure and surface decarbonization,which reduce toughness of the steel,particularly the flake martensitic structure that is most harmful to operation properties of the steel,and significantly reduces the steel’s impact toughness,being the direct cause for stress corrosion and rupture of the steel.