Formation of martensitic islands is one of the most significant causes of deterioration in toughness of multi-pass weld heat-affected zone of high tensile strength steels. This paper presents the influence of the size and the volume of the martensitic islands on the primary mechanical properties and the cleavage fracture toughness of steels simulated to intercritically reheated grain coarsened HAZ of multi-pass weld. Various thermal cycles simulated to HAZ were applied to five kinds of laboratory melted steels produced by hot rolling, direct quenching and tempering process to reproduce of different states of the martensitic islands. The states of the martensitic islands represented by the average and the maximum size, the volume fraction and the aspect ratio were investigated in correlation with the thermal cycles. The yield and tensile strength increased with the increase of the volume fraction of the martensitic islands. However, the toughness was so sensitive to the formation of the martensitic islands that small amount of the martensitic islands significantly decreased the toughness.