W-containing duplex stainless steels designed on the base composition of Fe-22Cr-5.5Ni-0.15N have been developed and the synergic effect of W and Mo on the resistance to localized and stress corrosion in chloride solution and on the formation of sigma phase and α' phase of the alloys were evaluated. The addition of small amount(about 1%) of Mo to Fe-22Cr-5.5Ni-3W-0.15N alloy significantly increases the resistane to localized and stress corrosion of the alloy in hot chlorides. This is associated with a synergic effect of W and Mo on the stability and repassivation rate of the passive film of the alloy in chlorides. However, it is found that the alloy is highly susceptible to the embrittlement, induced by the formation of α' phase during aging at 475℃. This embrittlement is due to the Mo content in the alloy. The susceptibility to α' phase embrittlement is decreased by the addition of the smaller content of Mo(about 0.5 wt.%) to the alloy, since Mo is more effective in increasing the formation rate of α' phase than W in duplex stainless steels.