The notch toughness of clad steels with various clad arrangements were studied previously, and it was clarified that the sandwich type clad steel with stainless steel at the center had the highest toughness as reported in the 1st and 2nd papers. In this paper, the effect of clad ratio, properties of base metal and plate thickness on the notch toughness of the sandwich type clad steel were investigated. The results obtained are as follows. 1) The toughness of clad steel is improved in accordance with increase of clad ratio. Increase of thickness of clad metal makes higher crack initiation energy level and critical propagation stress, while reduction of thickness of base metal lowers various transition temperatures. 2) The effect of properties of base metal on the toughness of clad steel is remarkable. Quenched and tempered clad steel of which base metal is HY-80 shows extremely lower transition temeratures than as-rolled clad steel of which base metal is mild steel. In this case, however, it is necessary to use low carbon stainless steel, hardely embrittled by heat treatment, as clad metal. 3) Various transition temperatures of clad steels with the same clad ratio tend to rise in accordance with increase of total plate thickness. 4) Comparatively close correlation exists between the arresting temperature by Double Tension test and the fracture transition temperature by Van der Veen test.