The microstructure and mechanical properties of Ti-5553 alloy bars with large diameter Φ300 mm were investigated. The result indicates that the influence of the solution temperature changes on mechanical properties is remarkable. When the Ti-5553 alloy is subjected to BASCA heat treatment, it reveals high damage-tolerant property including the fracture toughness up to 87.3 MPa·m1/2 and the strength up to 1100 MPa. After solution treatment at 820 oC and aging treatment from 580 to 620 oC, a large amount of primary equiaxial a phases and secondary acicular a phases distribute in the β matrix.?The mechanical properties of Ti-5553 alloy are determined by the relative volume fraction of primary a and secondary a phases.