Abstract: The research is focused on changes of the structure and flattening property of the hi-frequency welded tube after welding and hot-rolling/stretch-reducing. The research reveals that the hi-frequency welding process not only causes the difference between the structure of the metal under the thermal effect by welding and the structure of the base metal, but also makes changes of orientation and continuity of the metal flow line in the steel; after hot-rolling/stretch-reducing the said structure of the metal under thermal effect and structure of the base metal, both changing along with the increase of the deformation amount of the work piece become similar; change of the metal flow line orientation during welding and the significant amount of inclusions in the steel are responsible for the fact that during flattening test, the fracture quite often take places at the side of the test piece under thermal effect by the welding.