Abstract: To quantify the defects location inside butt welds in small diameter( 8 mm) tubes, parallax method was investigated. The quantification method was found after image analysis on the testing results, and the factors correlated with measurement precision of defects location were analyzed. The results showed that the defects location could be calculated with enough precision to meet the requirement after taking certain techniques, which included setting properly marks on the tubes, adopting projective magnification techniques.