This paper addresses the problem of theoretically predicting the evolution of void for a single crystal in ductile material accounting to the size and orientation effects. In this paper, a new damage model is derived based on the theory of mechanism-based strain gradient crystal plasticity (MSG-CP). By imposing the Taylor dislocation model into a widely used Gurson model (1), we extend the Gurson model to account for the void size effect. Meanwhile, we consider the crystal orientation effect by using MSG-CP to describe the behavior of matrix. Numerical simulation has been conducted under axisymmetric loading condition for cylindrical void and under spherical symmetric tension for spherical void . It reveals that the damage of a ductile porous material has strong orientationdependence and size-dependence on microscale level. The traditional conclusion that the larger the void size is the faster it grows is also verified by the new model. Additionally, we add a kinematic hardening law to the MSG-CP theory, and have analyzed a hysteresic response of a single crystal under cyclic loading.