The objective of this paper is to investigate the effect of residual stresses on the $Delta$K$sub$th/ and fatigue crack growth behavior of butt weldments. For this purpose, transverse butt sutmerged arc welding was performed on SM50A steel plate and CT(compact tension) specimens which loading direction is perpendicular to weld bead were selected. Welding residual stresses distribution on the specimen was determined by hole drilling method. The case of crack located parallel to weld bead, the states of as weld and PWHT, $Delta$K$sub$th/ of specimens(HAZ, weld zone) was higher than that of the base metal probably because of the compressive residual stresses of crack tip. In low $Delta$K region, it is estimated that the effects of residual stresses for da/dN are great. In region II, the da/dN of weldments in as weld state was lower than that of the base metal. Though da/dN of Weldments in PWHT state was similar to that of the base metal. The constant of power law, m in two states consisted with the base metal. Therefore , it is estimated that the value of m is not affected by residual stresses. Fatigue crack growth behavior of weldments consisted with the base metal considering the effective stress intensity factor range($Delta$K$sub$eff/) included the effect of initial residual stress(Kres). Thus, we can predict the fatigue crack growth behavior of weldment by knowing the distribution of initial residual stress at the crack tip.