Shear-box test with strain measurement was used to study time-dependent stress and strain of in-plane shear (Mode II) fracture process of rock and to reveal the mechanism of Mode II fracture. Numerical results show that the maximum shear stress τ _max at the crack tip is much larger than the maximum tensile stress σ ₁ and the ratio of τ _max/σ ₁ is about 5, which favors Mode II fracture of rock. Test results indicate that the strain-time curve comprises three stages: the linear deformation stage, the micro-cracking stage and the macroscopic crack propagation. The strain in the direction of the original notch plane is negative, due to restraining effect of compressive loading applied to the original notch plane. Both σ ₁ and τ _max are increased as the load increases, but the slope of τ _max is larger than that of σ ₁ and the value of τ _max is always larger than that of σ ₁. Therefore, τ _max reaches its limited value at peak load before σ ₁ and results in Mode II fracture of rock. Shear-box (i.e. compression-shear) test becomes a potential standard method for achieving the true Mode II fracture and determining Mode II fracture toughness of rock.