Non-quenched and tempered N80 tubing belongs to economical low alloy steel. Chemical composition, microstructure, mechanical property, fracture morphology and micro-area chemical composition of the brittle fractured tubing was analyzed. The results showed that the inhomogeneous cooling after finish-rolling or normalization and segregation of V element results in hard-brittle banded martensite formation in the tube, which significantly reduced the toughness of tubing. N80 tube brittle fractured from the banded structure of martensite. Failure tubing was re-heat-treated, eliminated martensite by controlling cooling rate, uniform fine pearlite and ferrite, and good toughness tubing was obtained.