Abstract: Causes for brittle fracture of the N80-1 tubing as occurring during hydraulic fracturing operation at oil reservoir are investigated by means of both macro and micro analyses on the fracture，and metallographic test，etc. The results indicate that the primary cause for the said fracture of the tubing is a 0.2~0.8 mm-deep scratch as made by the mandrel on the inner surface thereof. In addition， rather low impact toughness of the N80-1 tubing is also responsible for the brittle fracture. Accordingly，using the tubing with high impact toughness for hydraulic fracturing service will reduce the probability of brittle fracture.