This paper investigates hardness, toughness and microstructures of A572 Gr.50 steel built-up box-section welds jointed by electroslag welding (ESW). The welding heat from ESW, for example, is considered one of the main factors reducing steel toughness and causing the connections to fracture. The result of the investigation indicates that the 25- and 50-mm plates have small difference in hardness, toughness and grain sizes. The A572 Gr.50 steel absorbs energy more than 105J, satisfying the code requirement of 27J at 0°C in the Charpy impact tests. However, the welding heat increases the size of coarse grain and hardness in the heat affected zone (HAZ), reducing the Charpy value to 14.9 J. The HAZ absorbs much less energy than the base material, and this difference is more significant in the weld of 50-mm plates. Based on the investigation, it can be suggested that to enhance the seismic performance of welded steel structures, not only the toughness of base material, but also welding heat need further controlling.