This study proposed and validated a new method to evaluate the mode II interlaminar fracture toughness both at the initial delamination onset and during the crack growth stage by using a doubly end-notched tension (DENT) specimen. The specimen configuration idealized the growth of two mode II cracks in a double-lap joint. Shear-lag analysis indicated that the energy release rate of the DENT specimen is independent of the crack length and that the crack growth is stable. The initial mode II fracture toughness obtained by quasi-static tensile tests of the proposed specimens largely agreed with that obtained by the end notched flexure (ENF) tests. This trend was also valid in the other CFRP unidirectional laminates with different Young’s moduli. Furthermore, the present study demonstrated that the crack growth resistance curve (R-curve) of mode II delamination could be obtained by unloading-reloading tests of the proposed specimens.