The influences of very narrow weld bead and highly strength overmatching on the fracture toughness of laser beam welded components are investigated. This study focuses on the difference of plastic constraint in fracture toughness specimen (3-point bend specimen, 3PB) and structural components with crack in the weld metal. The equivalent crack tip opening displacement (CTOD) ratio, β, for the correction of CTOD toughness for constraint loss in structural components is numerically analyzed. The narrowness of weld bead induces constraint loss by plastic deformation in the base metal, both the 3PB and structural components. Nevertheless, in the case of the wide plate with shallow crack, the highly strength overmatching elevates the opening stress. Therefore, it is found that the β increase with increasing strength mismatch ratio. The influence of highly strength overmatching on β is important for assessment of fracture performance of laser beam welded component. The applicability of β to correction of critical CTOD in laser beam welded components is discussed. The fracture toughness tests with the 3PB and tension panel (edge through-crack panel) of the laser beam welded joints were conducted at a low temperature in the brittle fracture range. It is indicated that β can be applied to the CTOD toughness correction in laser beam welded component.