Abstract: Some steel tube used in oil well was leakage in the period of several months. By means of the chemical composition analysis, mechanical preperties testing, microstructure examination, ultrasonic testing (NTD), water chemical composition analysis and SEM energy spectrum analysis, the leakage steel tube was analyzed. The results showed, that tube was welding tube, it was′t the seamless steel tube. Because of the quality of the welding tube was very bad, and the corrosion of the medium, the welding tube was leakage in a short time finally.