The degradation of the fracture toughness of high strength low alloy steels is attributed to the formation of ‘local brittle zones’ in the welded joint. These local brittle zones are mainly located within the coarse grained heat affected zone (CGHAZ) and the intercritically reheated CGHAZ (ICCGHAZ). Cracking and debonding of M-A constituents from the surrounding matrix are generally accepted as initiation events of fracture in the ICCGHAZ. In the present work, the low temperature fracture toughness of X80 pipeline steel was examined. The main purposes were (i) to evaluate possible crack initiation sites of cleavage fracture and (ii) to identify the mechanism by which M-A constituents deteriorate the ICCGHAZ toughness. The results revealed that the microstructure of ICCGHAZ contained blocky M-A constituents along prior austenite grain boundaries. Finally, it was shown that fracture initiation occurred preferentially at M-A constituents by a debonding mechanism rather than cracking of the M-A constituents.