In order to predict the sigma (σ) phase embrittlement in multipass weldment of duplex stainless steels, the precipitation behaviours of the σ phase in the base metal and the weld metal were investigated using super duplex stainless steels (SAF2507 and DP3W) and a conventional duplex stainless steel (NAS64). In all the steels, the amount of the σ phase precipitated during an isothermal ageing process sigmoidally increased with the length of the ageing time. The kinetics of σ phase precipitation in the base metal and the weld metal could be expressed approximately by the Johnson-Mehl type equation, however, the initiating time of σ phase precipitation in the weld metal was longer than that in the base metal. The precipitation of the σ phase in the DP3W base metal was much delayed and depressed compared with the SAF2507 and NAS64 base metals. Increasing the amount of the σ phase drastically decreased the absorbed impact energies of aged steels. They fell to as low as 1/4 of the unaged values when the amount of the σ phase exceeded 5%. The precipitation of the σ phase in duplex stainless steels during multiple thermal cycle process were predicted using the additivity rule. We predicted that the largest amount of the σ phase and the most significant degradation of impact toughness would occur in the base metal region (HAZ) for peak temperatures of 1 100–1 300 K in multiple thermal cycle.