Abstract: In this paper weld thermal simulation technique was introduced to study the effect of boron on Charpy impact energy and fracture toughness COD of the weld coarse,grained HAZ in Cr-Ni-Mo low carbon-low alloy steel.The results show that the upper shelf transition temperature of Charpy impact energy curve increases,the fracture toughness COD decreases and the brittleness increases both in base metal and in the coarse grained HAZ with the increasing of boron content.This is considered to be related to the precipitation of borocarbides in the grain and along the grain boundaries,and to the solid solution of boron.