A co-operative investigation was made by a task group of ASTM Committee E-24 Subcommittee II to correlate the microfractography and macrofractography of notch tension specimens of 4340 steel heat treated to 8 strength levels. Selected testing temperatures provided fractures with macroscopic topography representing the complete fracture transition of each material. The observed microscopic fracture modes were correlated with the two macroscopic zones of fracture surface configurations (fibrous and radial) and the final area of separation. Regardless of tempering temperature the fibrous zone of all materials consisted of microvoid coalescence and holes. Radial zones consisted of quasi cleavage for specimens tempered at high temperatures, and intergranular fracture with grain boundary microvoid coalescence for specimens tempered at low temperatures. Mixtures of these modes were observed in radial zones on specimens tempered at intermediate temperatures. The amount of each of these modes present in the radial zone depended on both testing and tempering temperatures. This correlation and the strong temperature dependencies become evident by the examination of fractures representing the complete macroscopic transitional appearances of sufficient materials obtained by different tempering temperatures.