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High strength steel plates (950MPa class strength) have recently been studied as material to reduce THalnifacturing cost in hvdropower plants. This steel has already been used as penstocks in the Kanna-gawa and Omaru-gawa hydropower plants in Japan. High strength steel, however, has welding restrictions in that pre-heating, post-heating or inter-pass temperature range for preventing weld cracking and keeping joint strength are much more severe than the restriction of ordinary strength steel. One of the countermeasures against welding restrictions is undermatching (lower strength in the weld metal than in the base metal) by using softer welding materials. The usefulness of undermatching in HT950 welded joint was clarified from the viewpoint of joint strength and fracture assessment in a previous study. In this paper, the effect of welding imperfections, such as mechanical heterogeneity, weld geometry and the stress - strain field, on joint strength and fracture assessment in undermatched welded joint was investigated by numerical analysis using finite element method (FEM). As results, mechanical heterogeneity in undermatched welded joint was characterized by softened weld material and thermal cycles at the heat-affected zone (HAZ). Welding-induced residual stress and distortion were evaluated by a thermal elastic-plastic analysis. Based on these results, the effect of welding imperfections on joint strength and fracture assessment in an undermatched welded joint was investigated. It was concluded that undermatched welded joint with welding imperfections allowable by technical indication shows almost equal strength and fracture resistance to that of undermatched welded joint without welding imperfections.