Abstract: The study evaluated the micro indentation size effect of a Zr55Cu30Al10Ni5 bulk metallic glass. Firstly, a conventional theory of mechanism-based strain gradient plasticity (CMSG) for pressure-sensitive materials was built based on the Drucker-Prager yield criteria. Secondly, the finite element format of the constitutive relation of the CMSG theory was set up using the user-defined material subroutine (UMAT) interface of the commercial software ABAQUS. Thirdly, the conical indentation test response was simulated and analyzed via the subroutine UMAT and verified by experiments. The results show that CMSG theory can accurately describe the elastic-plastic behavior of the materials. Finally, the load-displacement curve and hardness of the material under different indentation depths were studied on the basis of the previous results. It is clear that the hardness of metallic glass increases with indentation depth decreasing, indicating that the CMSG theory can predict the size effect of the metallic glass Zr₅₅Cu₃₀Al₁₀Ni₅ in micro scales. Furthermore, analysis of load-displacement curves under different friction coefficient demonstrates that the micro indentation response of the material is hardly affected by friction.