This study concerns with welded connections of high strength steel H-SA700. Since the strength of specified welding material for H-SA700 steel is less than the strength of base metal, the method of joining the high strength steel by undermatched welding is studied. In order to verify the strength of fillet weld joints for built-up members made by undermatched welds, loading tests of fillet welded joints and beam-to-column subassemblages were conducted and the validity of proposed methods to evaluate strengths of welded connections are examined.