Toughness, especially ductile-to-brittle transition temperature (DBTT) such as fracture appearance transition temperature (FATT) obtained from Charpy impact test, is one of the most important properties of steels to assure material reliability. One of determining factors of DBTT in ferrite-pearlitic and martensitic steels is well known as the effective grain size (d(EFF)) on cleavage fracture surfaces. However, in the steels with intermediate stage transformation microstructures (Zw), relationship between DBTT and d(EFF) has not been clarified because of the complicated microstructures. Meanwhile, absorbed energies of Charpy impact test has been standardised for steel applications. The present study aims to determine the relationship between DBTT and d(EFF), and the relationship between upper shelf energy (USE) and plastic properties in a quenched and tempered low carbon high strength steel which has a Zw microstructure consisting of granular bainitic ferrite and quasi-polygonal ferrite. Size distribution of d(EFF) was measured on the cleavage fracture surfaces revealed by Charpy impact testing at 77 K, and correlation between d(EFF) and the microstructure was examined. It was found that FATT is inversely proportional to In (d(EFF)(-1/2)) with a slope close to that approximated for ferrite-pearlitic steels. Investigation of the correspondence between cracks and EBSD grain boundaries revealed that d(EFF) apparently agrees with the grain size of bainitic ferrite enclosed by large angle grain boundaries with misorientation over 15 degrees. Moreover, USE has been proportional to total plastic works until fracture of tensile test. These results indicated that the absorbed energies at any temperatures could be computable.