Recent validation of steels for offshore structures is often determined by the crack tip opening displacement (CTOD) value, which is very sensitive to the local brittle zones (LBZs) in weld heat-affected zone (HAZ). Improving toughness of the LBZs is the purpose of this study that discusses the morphological effect of a very hard phase in the LBZ [e. i. the martensite-austenite constituent (M-A)], on their toughness. Morphological factors of interest are the volume fractions of both M-A particles in total ( V_f ) and the slender M-A particles ( SV_f ) which were defined as those with a larger aspect ratio than four. The analyses were made with a binary image processor for steels of various chemistries and HAZs through various heat cycles. The volume fraction of the slender M-A ( SV_f ) affected the LBZ toughness; the toughness was degraded remarkably with increase in the SV_f value but this harm of the slender M-A particles was eliminated as the SV_f value exceeded a particular value. Frequent observation of the boundary breaking between an M-A particle and the matrix in the vicinity of the principal crack tip indicated that the interfacial cracking is preferential on the primary stage of the fracture. The stress and the strain characteristics in and around the local hard zone (LHZ) in a soft matrix were figured out by the elastic and plastic finite element analyses. The slender LHZ with a larger aspect ratio lies under the condition of the greater strain-difference between LHZ and matrix as well as the rise of the stress in LHZ. However, these increases in the strain-difference and the inner stress are suppressed when LHZs come closer together (vicinity effect). These calculated results explained the unique behavior observed in the experiment as the following : The rise of the SV_f value increases the possibility of fracture both at the LHZ/matrix interface and in LHZ itself associating with the large strain-difference and the escalated inner stress. Simultaneously, the increase in the SV_f value shortens the distance between M-A particles so that the vicinity effect works and decreases the possibility of fracture.