Abstract: Through relevant NDT process, surface hair cracks are found with seamless steel linepipes as used for several gas pipelines that have been operating in a certain oil field for about one year. Based on test analysis of the hair cracks, it is identified that the hair cracks are of some kind of fold defect with depths within the tolerance range as specified in application specification, and that such defects are ex-works ones, and have not extended during the site operation. No defects beyond the application specification are found with the lot of seamless steel linepipes by relevant visual inspection, and only after undergoing surface grinding treatment, and then NDT check, are the said hair defects revealed. According to the applicable Specification GB/T 8163-1987 used for the order, this lot of linepipes are in compliance with relevant technical requirements.