It is well known that coarsening of carbides, intergranular impurity segregation and other microstructural change degrade the fracture toughness of ferritic low alloy steels after long service operation. The ductile-brittle transition temperature (DBTT) odtained by Charpy V-notch impact test and its shift after service operation is one of the most useful measures of the fracture characteristics of these materials. However, it is difficult to extract multiple specimens for CVN tests without damaging the components. In this study, the ductile-brittle fracture mode transition behavior of ferritic low alloy steels was investigated by means of a small punch test technique in relation to CVN DBTT behavior. The experimental results revealed a single linear correlation between the DBTT obtained from small punch test and that obtained from a Charpy V-notched impact test for various kinds of ferritic low alloy steels.