This study investigated the microstructures and mechanical properties of lower bainite and tempered martensite in JIS SK5 steel. At equivalent hardness, the toughness and ductility of lower bainite are superior to those of tempered martensite. However, the lower bainite has a lower yield strength owing to that the bainite sheaf is larger than the tempered martensite plate. The fracture surface of lower bainite exhibits transgranular cleavage and differs considerably from that of tempered martensite. Tempered martensite embrittlement (TME) occurred in the tempered martensite, which is dominated by intergranular failure. It is caused by grain boundary segregation of phosphorus and grain boundary precipitation of carbide during tempering. Additionally, the size of the cleavage facet of lower bainite was demonstrated to be correlated with the width of the bainite sheaf. The results of electron back-scatter diffraction (EBSD) analysis indicates not only that the sheaf boundary is a high-angle boundary, but also that the cleavage crack travels along the { 0 0 1} ferrite plane, whose surface energy is low. (c) 2007 Elsevier B.V. All rights reserved.