Thermal stress can trigger slip in rock fractures. Understanding this thermally induced slip behavior of fractures in crystalline rock is crucial for the design and operation of enhanced geothermal systems, and deep geological repository. The present study aims to investigate mechanism and mechanical response of thermal-induced slip in a single fracture. We develop a FEM-based model to simulate the thermally induced slip in fractures with different surface roughness, and under different boundary conditions. The model is validated against a unique set of thermally induced slip test data in laboratory. The results show that thermal stress can cause a decrease in normal stress on the fracture surface, accompanied by an increase in shear stress. The mechanical boundary conditions and fracture roughness can significantly influence the fracture aperture evolution during the slip process. Specifically, results show that increasing the vertical stress can induce a transition from fracture closure to dilation (3.8% closure at z= 1 MPa versus 23.7% aperture increase at z= 50 MPa), while a higher normal stiffness suppresses slip and limits aperture changes. Additionally, rougher fractures exhibit greater closure (the roughest fracture exhibits 3.8% closure vs 1.0% for the smoothest), and neglecting fracture surface plasticity leads to an underestimation of fracture closure in the slip process.