To explore the micro-mechanism for the size effect of the mechanical behavior of metallic crystals, the tensile and fatigue behavior of [345] Cu single crystal with t of 0.1−2.0 mm is investigated. The results show that with the reduction of t, an obvious increase in σ YS and a slight decrease in σ UTS take place; meanwhile, the δ evidently reduces, especially as t < 0.6 mm, while the N f first sharply increases and then decreases at a constant stress amplitude of 80 MPa. The activated slip system reduces with t under tensile loading, and the fracture modes are transferred from ductile to slip separation rupture, whereas under cyclic loading, slip separation rupture becomes dominant especially at t = 2.0 and 0.1 mm. Correspondingly, the tensile microstructures are transformed from the cell-walls to dislocation cells, while dislocation cells are the dominant microstructure in fatigued specimens, and the dislocation density obviously decreases with decreasing t. In a word, a strong dependence of tensile and fatigue behavior on t is exhibited for the [345] Cu single crystal.