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In this work, the anisotropic microstructure and mechanical properties of selective laser melted (SLMed) Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55511) alloy before and after aging treatment are investigated. Owing to the unique thermal gradient, the prior columnar β grains with {001} texture component grow in the building direction, and the mechanical properties of the as-fabricated Ti-55511 alloy exhibit slight anisotropy. Aging treatment creates uniform precipitation of the α phase at the boundaries as well as the interior of β grains. Due to the microstructure of the aged samples with a weak texture, the mechanical properties exhibit almost isotropic characteristics with an ultimate tensile strength of 1133 to 1166 MPa, yield strength of 1093 to 1123 MPa, and elongation from 13 to 16%, which meet the aerospace allowable specification very well. By XRD and EBSD analyses, the total dislocation density of the aged samples (~134.8 × 1013 m−2) is significantly lower than that of the as-fabricated samples (~259.4 × 1013 m−2); however, the aged samples exhibit a higher geometrically necessary dislocation (GND) density (~28.5 × 1013 m−2) compared with the as-fabricated samples GND density (~2.9 × 1013 m−2). Thus, a new approach to strengthening theory for estimating the anisotropic mechanical properties of AM alloys is proposed.