This research studied the fracture toughness of the Fe-7Al-27Mn alloys with increasing carbon contents : 0.5% C, Fl alloy : 0.7% C, F2 alloy (with 4.0% Cr); and 1.0% C, F3 alloy. Fracture toughness experiments were conducted at temperatures of 25, - 50, - 100 and - 150 degrees C. It was found that plane-stress, K-C, values as measured by the R-curve method, decreased as the temperature dropped. F1 alloy possessed the highest K-C value at all temperatures among the three alloys. The K-C values of the F2 and F3 alloys were similar at ambient temperatures, but F3 maintained the toughness property and ductility better at sub-zero temperatures. Quantitatively, K-C va lues of the F2 alloy at - 150 degrees C were ca. 60%, less than at 25 degrees C, but F1 and F3 alloys dropped by only ca. 30%. Using a compact-tension specimen, 20.0 mm thick, at - 150 degrees C only alloy F2 satisfied the requirement of plane-strain fracture toughness with a K-IC value of 106 MPa m(1/2). The existence of Cr (4.0%) and the formation of a ferrite phase in an austenite matrix was responsible for the tow toughness value observed.