To prevent brittle fractures in thick plate steel structures, the fracture toughness of butt welded joints fabricated from 150 mm thick Q345B steel plates were tested at low temperature by three-point bending specimens and crack tip opening displacement (CTOD) was applied as a fracture toughness index. The relationships between fracture toughness CTOD δm and temperature for weld metal and heat affected zone (HAZ) were analyzed through Boltzmann curve fitting from toughness versus temperature data. Furthermore, the fracture mechanism of the tested specimens were investigated with scanning electron micrographs. The test results indicated that the fracture toughness CTOD values δm for weld metal and HAZ reduced as the temperature decreased. From 20 ℃ to -60 ℃, the CTOD values δm for weld metal and HAZ reduced by 78% and 91%, respectively. Compared to the base metal and weld metal, the CTOD values δm for HAZ were much lower. In this study, rich experimental data were collected for fracture toughness of thick plate butt weld, so as to form the technical basis for preventing brittle fracture of thick plate steel structures in cold regions.