Several tests including CTOD and Charpy impact have been performed on the weld joints of high strength low alloy steel S335G10+N to identify the toughness. The fracture surface and microstructure of samples extracted from the weld joints are characterized using scanning electron microscopy. Based on all data obtained some relationships between toughness and microstructure has been detected. (1) The tendency of Charpy impact energy distribution with the position of notch is similar to the CTOD values in WP and FL but different in FL+2.CTOD values of FL+2 are greater than those of FL, but the Charpy impact energy has an opposite tendency. (2) The appearance of Pearlite and upper bainite found in WP sample S3554 is regarded as one of the reasons for the lowest CTOD values. The cavity and quasi-cleavage in S35510 fracture surface indicate an inferior toughness compared with some dimples and ripple morphology of slip separation in S3552. (3)Grains close to the fracture surface in the 35511 (Stable crack growth) of FL +2 were elongated owing to plastic deformation caused by crack growth. (4)Hardness of the HAZ is equal to fusion zone and lower than base metal or weld position in S35511. Hardness of fusion zone increases with the depth. (5)A big difference of Average Grain Size is found in microstructure close to the fracture surface among S3557, S3556 and S3559.The greater the Average Grain Size is, the lower the CTOD value will appear.