The microstructures and fatigue crack growth rates (FCGRa) of EH36 TMCP steel base plates and welds were investigated. A minor increase in hardness in the as-welded heat affected zone (HAZ) can be attributed to the low carbon equivalent (CE) of the steel. Hence, the improvement in the HAZ toughness of the weld, as compared to the traditional hardened steel weld, can be anticipated. Microstructural observations indicate the coarse-grained HAZ is composed of a large amount of lower bainite with some upper bainite. Fine-grained HAZ consists of refined ferrite and bainite, together with interlath microphases. Although there is an obvious different impact energy values for L and T specimens, the effect of orientation and tempering treatment only has a slight influence on the FCGRs of the TMCP steel plate, as compared the data obtained from LT, TL and tempered specimens. The lower FCGRs of the HAZ than those of the steel base plate, regardless of the post-weld heat treatment (PWHT), can be attributed to the formation of low-carbon bainite with high toughness, instead of ferrite and intergranular pearlite in the steel plate. The as-welded fusion zone in a CW specimen shows a significantly high Paris-law gradient. The evidence shows that the TMCP steel weld after tempering at 600.degree.C for 2 hr possesses a slightly better resistance to crack growth than the steel plate.