Abstract: Microstructure, properties and boron distribution of HAZ in an ultra-low carbon bainitic steel bearing Cu-Nb-B were investigated. Results indicated that excellent low temperance toughness could be obtained the HAZ. The influence of cooling time (t₈₅) from 800 to 500℃ on bainitic morphology and the effect of microstructure on low temperature toughness were discussed. When cooled at medium rate, a part of granular bainite forms at higher temperature and then the most pan of austenite transforms into a finer lath bainite forms at lower temperature, this microstructure has better low temperature toughness.