Fatigue crack growth rates for the base metal and welds of SM50A steel weldments were determined at temperatures ranging from room temperature down to 123 K. For the base metal, crack growth rates between room temperature and 188 K decreased slightly with decreasing temperature. At 123 K, the growth rate of the base metal increased markedly because of cyclic cleavage during striation growth. For the welds, no systematic correlation can be made between temperature and growth rate. The growth rates of the welds were lower than those of the base metal regardless of temperature because the crack tips in the weld specimens always existed in the field of compressive residual stresses. Therefore the growth rates of the welds were dominated by residual welding stress rather than temperature. The crack growth rates of the base metal and the welds with residual stresses under low temperatures were closely correlated with the effective stress intensity factor range (ΔK_eff), estimated based on crack closure measurements, when the crack growth mechanism was dominated by striation formation.