The influence of Mo substitution by W on impact toughness was investigated and related to the microstructure in duplex stainless steels. W had a lesser pronounced effect on promoting embrittlement than Mo. only sigma phase was observed in conventional alloy containing 3% Mo. However, as Mo was substituted by W chi and/or Laves phases were formed besides the sigma phase. Especially Laves phase was formed intragranularly within austenite grain. The sigma phase formation was also the main reason of embrittlement although Mo was fully substituted by W. However, the Mo substitution by W significantly delayed the nucleation of sigma phase and also decreased the number of its nucleus. Therefore, the increasing rate of sigma phase amount was remarkably slowed down. Chi phase nucleated prior to the sigma phase and occupied preferential nucleation sites, grain boundary triple junctions. One hour aging treatment, causing chi phase formation but virtually no sigma phase, resulted in a little reduction of impact toughness.