Abstract: To elucidate the anisotropic mechanisms governing microstructure and mechanical properties of Ti-6Al-4V alloy formed by selective laser melting (SLM) in the as-deposited state, this study systematically compared microstructures and tensile properties between the XY and XZ planes through tensile testing, X-ray diffraction (XRD), and microstructural analysis. Results indicate that both planes consist of α′-Ti phase and trace β-Ti phase. However, primary β grains in the XZ plane exhibit a columnar structure with fine needle-like α′ martensite distributed internally, whereas β grains in the XY plane are near-isotropic. The XZ-plane specimen exhibited a tensile strength of 1207.2 MPa and an elongation after fracture of 4.62%, demonstrating a “high strength-low ductility” characteristic with a brittle-ductile mixed fracture mode. The XY-plane specimen showed a tensile strength of 1169 MPa and an elongation after fracture of 7.3%, exhibiting ductile fracture behavior. This study reveals the mechanical anisotropy behavior of SLM-formed Ti-6Al-4V alloy in the as-deposited state, attributed to the distribution of columnar grains and martensite. It provides experimental evidence for understanding the microstructure-property relationship in the as-deposited state and for optimizing subsequent heat treatment processes.