Abstract：Analytical solution of instability criterion for fault rockburst is proposed using shear strain gradient plasticity theory and energy criterion. Fault band is treated as one dimensional shear problem and the thickness of the band is dependent on characteristic length of rock. Dissipated potential energy and post-peak stiffness of fault band are deduced and both of them are concerned with the thickness of fault band. To propose an instability criterion，half of fault band and half of elastic rock outside the band comprise a system to which energy criterion is applied. The criterion shows that stability of the system depends on the constitutive parameters and structural size. So，the criterion exhibits size effect. Present instability criterion for fault rockburst is analogous to coal pillar rockburst criterion，for lower ratio of shear elastic modulus to shear softening modulus and lower ratio of thickness of fault band to size of elastic rock outside fault band lead to instability of the system. The advantages of present investigation for fault rockburst based on gradient-dependent plasticity are that thickness of fault band，dissipated potential energy，post-peak stiffness and instability criterion can be analytically presented，and plastic shear strain is localized into fault band，which is consistent with field observations.
