Room temperature compression performance of polysynthetic twinned crystal (PST crystal) in Ti－45Al－8Nb (atomic fraction, %) alloys with different angles between lamellar orientation and uniaxial stress direction was studied. The causes of the fracture failure were investigated by the fracture surface and the effects of the Nb element on strengthening were also studied. It was found that when the lamellar orientation in the PST crystal was paralleled or vertical to the stress direction, shear deformation was vertical to lamellar interface, and when the angle of lamellar orientation to the stress direction was 45°, the shear deformation was paralleled to lamellar interface. The yield strength of the 0° PST crystal was the maximum which was up to 1296 MPa. The PST crystal of 45° had the best compression plastic strain of approximately 1.1%, but its yield strength was the minimum of only 847 MPa. Nb element could refine lamellar structure and improve the room temperature yield strength significantly through increasing shear stress of dislocation motion and reducing the stacking fault energy.