Abstract: The model of electric resistance welding (ERW) tube, which reflects welded seam, heat affected zone and base material, was built up in finite element simulation. The ratio of strain increment was adopted as a necking criterion of tube in hydroforming numerical simulation. Furthermore, using the above method, the forming limit curve of QSTE340 ERW tube was determined. The corresponding forming limit test of QSTE340 ERW tube was completed using the processing path calculated form numerical simulation. Compared with the experiment the forming limit curve obtained by this method is in agreement with the experimental data. It is proved that the hydroforming forming limit of welded tube calculated based on numerical simulation is believable.