The Ni-based single crystal superalloys are widely used in key hot section parts of advanced aero engine due to the superior high temperature mechanical properties. Multi-axial stresses resulting from complex temperature and stress state happen frequently in blades during service, thus the mechanical properties of three orientations need to be studied. However, most of these works are conducted in the first and second single crystal superalloys and there is rare report concerning the third single superalloys. Therefore, in this work the microstructures and tensile properties of the third generation single crystal superalloy DD9 with [001], [011] and [111] orientations were investigated by OM, SEM, TEM and tensile testing machine at 760 and 1100 ℃. The results show that as-cast dendritic structures and heat treated γ’ of DD9 alloy with three orientations are different on the section perpendicular to the crystal growth direction. With rising of temperature, the ultimate tensile strength and yield strength decrease and tensile anisotropy drops obviously. The ultimate tensile strength and yield strength of DD9 alloy with [001] orientation are higher than those with [011] and [111] orientation except that the yield strength with [001] orientation is slightly lower than that with [011] orientation. With temperature increasing, the fracture characteristic transforms from quasi-cleavage at 760 ℃ to dimple at 1100 ℃. At 760 ℃, very high density dislocations appear in the matrix channels with [001], [011] and [111] orientations, but some stacking faults are present only in γ’ particles with [001] orientation. At 1100 ℃, the high density dislocation networks resulted in the matrix channels and particles of the alloy with [001] and [111] orientations, while a large number of deformation twins are found in samples with [011] orientation.