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Three-point bending test and acoustic emission technique are used to determine the fracture toughness and fracture process of three types of modified 9Cr-1Mo steel weld metals. Scanning electron microscopy is used to accomplish fractography analysis of fracture specimens. Microstructure of weld metals is investigated using optical microscopy and transmission electron microscopy. The fracture process and factors which affect fracture of the 9Cr-1Mo steel weld metals in post-weld heat treated condition are studied. Experimental results show that the modified 9Cr-1Mo steel weld metals fracture by a quasi-cleavage mechanism at ambient temperature. The microstructure of the weld metals is composed of mainly tempered martensite with M23C6 precipitates. In weld metals, microcracks nucleate from non-metallic inclusions. Fractures develop very quickly when cracks started to propagate. Comparatively, in weld metals with low strength, microcracks initiate at a low stress, but propagation of cracks is limited by plastic deformation. In weld metals with high strength, microcracks nucleate at high stresses, but cracks propagate very quickly and lead to almost immediate fracture of the specimens. As a result, weld metals with the low strength have a higher fracture toughness, while weld metals with higher strengths has a lower fracture toughness. (C) 1999 Elsevier Science S.A. All rights reserved.