Abstract: Objective The aim is to study microstructure and mechanical properties of laser welded joints of TC4 titanium alloy. Methods By combining experiments and testing methods, welding experiments of 2.5 mm thick TC4 titanium alloy were carried out using fiber laser welding method. By means of optical microscope, scanning electron microscope, universal tensile testing machine, and other testing methods, microstructure and mechanical properties of laser welded joints were investigated. Results The results show that weld is composed of acicular martensite α′ with coarse grains, which contains small secondary α′ phase in different directions. Microstructure of heat affected zone near weld is composed of α′ + α phases, microstructure of heat affected zone near base metal is composed of initial (α + β) + α phases. Transition zone between the two is composed of a small amount of α′ + α phase of high temperature β phase’ transformation + initial (α + β) phase. Tensile strength of TC4 titanium alloy welded joints is 1 048～1 058 MPa, and elongation of laser welded joints is 8.2%～9.8%, which are lower than that of base metal. Conclusion Both the front and back sides of weld appears silver-white, with a smooth and uniform surface. And both inside and outside of weld is free of defects, showing an I-shaped weld. Tensile fractures are characterized by ductile dimples, which indicates ductile fracture. Hardness of weld of welded joints is the highest, and hardness of heat affected zone is the lowest.