Electric resistance welding (ERW) is conventional manufacturing technology for production of linepipe. ERW is known for its low cost, high efficiency & reliability. Investigation was performed on Electric Resistance Welded (ERW) pipes having Ultrasonic Test (UT) indication observed during in-process inspection. The location of indication was adjacent to weld fusion line. The study was done on pipe size 168 mm (6.6 inch) outside diameter and 6.4 mm wall thickness of API 5L X52M grade to verify the cause of UT failure related to pipe manufacturing process and/or steel making process. The samples were drawn from UT indicated area and analysis is carried out by chemical testing, optical microscopy and scanning electron microscopy with energy dispersive spectroscopy. From systematical research by Why-Why analysis, it is clear that the reason of sub-surface defects on ERW pipe is due to presence of non-metallic inclusions. On the basis of the composition of observed non metallic inclusion, possible sources are identified. The countermeasures for each source cause were verified and successfully implemented the affected one.