The microstructure and toughness are investigated to elucidate the influence of boron on toughness in the high heat input welds of high strength low alloy(HSLA) steel. The result indicates that appropriate addition of boron effectively promotes the formation of acicular ferrite, and as a result microstructure is refined. Whereas the percentage of acicular ferrite decreases and microstructure is coarsen as the boron content further increases. Meanwhile, moderate amount of boron obviously suppresses proeutectoid ferrite, and bainite is favored. The fraction and size of M-A constituent accordingly increases from 2.4% and 2.14 μm with no addition of boron to 5.7% and 2.83 μm with 0.008 8%B. As result, the weld toughness is improred at first and then detoriorat as the boron overdoses. When the boron content is 0.005 2% with the titanium content of 0.03%, the highest toughness can be attained at low temperature for the high heat input welds of HSLA.