Abstract: By means of fracture analysis,chemical composition analysis,mechanical property test and metallography examination,the fracture reason of drill collar of oil well was analyzed. The results showed that the strength of drill collar was reduced because of abnormal big inside diameter. The drill collars were early fatigue fractured because bend load borne on drill collar was increased due to severe dog-leg of well hole,and torsion load borne on drill collar was added due to pipe sticking.