•True Mode II fracture toughness (KIIC) of anisotropic rock is measured by shear-box test.•Shear-box test is proved to be an effective method for measuring KIIC of anisotropic rock at any anisotropy orientation angle.•A new physical factors YI0 and YII0 are proposed to calculate KI0 and KII0 for shear-box specimen.•When θ = 0–10°, KIθ is negative, which promotes occurrence of Mode II fracture.•The KIIC of anisotropic shale is increased monotonously as β (β = 0–90°) increased. Fracture toughness of anisotropic shale is an important parameter in shale-gas exploitation technology. Currently available literature is mainly focused on Mode I fracture toughness (KIC) of anisotropic rock under tensile loading. Although there are the cracked straight through Brazilian disk and crack ring disk used to determine Mode II fracture toughness (KIIC) of anisotropic rock under pure shear stress, their fracture trajectories are deviated from the original crack plane and cannot be regarded as the true Mode II fracture. In this paper, shear-box test was firstly adopted to measure KIIC of anisotropic shale. New physical factors YI0 and YII0 were proposed to describe effects of both the geometry and the material parameters on stress intensity factors (SIFs) of the original crack plane (KI0 and KII0) and to derive the calculation formulae of SIFs on arbitrary (KIθ and KIIθ). Calculated results show that KIIθ reaches its maximum absolute value at θ = 0–10° where KIθ is negative, which promotes occurrence of Mode II fracture. The predicted planes of Mode II fracture agree well with the tested fracture trajectories (amostly along the original crack plane). The KIIC is increased with increase of β (β = 0–90°). KIIC is 3–4 times as large as KIC and can be regarded as the true Mode II fracture toughness of anisotropic shale. The shear-box test is an effective method for measuring KIIC of anisotropic rock.