Experimental investigation was performed to study the effect of front notch geometry and specimen dimensions in double torsion testing of 2124-T851 aluminum alloy. The results showed that the machined front notch geometry has no appreciable effect on the fracture toughness values, provided that the critical load is read off correctly from the load displacement curve, that is when a stable crack front has been fully developed. The results also showed that the double torsion test can yield valid fracture toughness values in aluminum 2124-T851, provided that the specimen thickness between bottoms of grooves is larger than 4.4 times the plane stress radius of plasticity and the overall thickness of the specimen is sufficient to avoid significant torsional warping.