The aim of this study is to improve the interlaminar fracture toughness of flax/gliadin composites. Some key parameters are used such as processing conditions, matrix plasticization, and fiber treatment. These parameters affect the crack-growth resistance since they are, respectively, related to the cross-linking density by varying the cooling conditions, to the reduction of the brittleness by adding a low amount of plasticizer, and to the quality of the fiber–matrix interface. Results show that interface quality is more dominant than the effect of a low amount of plasticizer for the initiation fracture toughness value. While crack initiation is closely related to weak links such as the fiber–matrix interface, crack propagation appears to be mainly in the matrix.