The engineering method for fracture toughness evaluation of beam-to-column weld HAZ using one-bead welds, called one-bead weld method, has been examined in this study. Materials used are 26 structural steels of 400 to 490 MPa strength class produced by different milling conditions. One-bead welds are made by CO2 arc welding with single bevel groove under heat inputs of 4.0 to 5.0 kJ/mm. From these welded joints, the Charpy specimens and 3-point bend CTOD specimens with the same thickness of 10mm are extracted. The notch is located at the weld HAZ on a straight bond side perpendicular to the plate surface. Charpy tests and CTOD tests are carried out at 0°C and -40°C, respectively, where the CTOD test temperature is determined on the basis of WES2808 correlation formula. It has been found that the Charpy absorbed energy is affected by the chemical composition of the base steel, such as C, Mn, P, S, N, and Ti. Based on similar test results, an engineering parameter, fHAZ, is proposed by the Building Center of Japan for estimating the HAZ toughness. However, the present study indicates that the total amount of N is not a proper measure, but the free N controls the HAZ toughness. From the fracture mechanical point of view, the correlation between the Charpy energy and the critical CTOD is useful. It is noted that the CTOD test results are sensitive to the length of CGHAZ along the crack front. The Charpy-CTOD correlation with respect to the HAZ toughness should be made with attention to the base-metal chemical compositions as well as the CGHAZ size along the crack front.