Oxygen enriched α composite layer with 82 μm thickness was formed on the surface of Ti-10V-2Fe-3Al alloy by the heat treatment at 830 oC for 1 h in air and water quenching. The effects of α composite layer on the microstructure and mechanical properties of Ti-10V-2Fe-3Al alloy were investigated. The microstructure, the hardness and the elements distribution of α composite layer were studied. The results show that the hardness variation of α composite layer does not always display a decreasing trend from the edge to the matrix, but shows the law of high-low-high to stable trend, which is related with the distribution of elements (especially V and Fe) and microstructure evolution. The surface hardness of Ti-10V-2Fe-3Al alloy with α composite layer increases by 45%; meanwhile, the tensile strength and yield strength decrease by 5% only. Cracks will be generated firstly on the sample surface along the direction perpendicular to the tensile stress in tensile procedure, then expand in α composite layer and cross up to the matrix material. Fracture morphologies show the characters of ductility in matrix zone and brittle fracture in α composite layer zone. Stress induced phase transformation from β phase to α″ phase will take place during the tensile deformation.