Abstract: The TC4 alloy samples with different dimension(ϕ8 mm×25 mm, ϕ25 mm×8 mm) were fabricated by electron beam selective melting forming technique. The evolution of microstructure and hardness along the powder deposition direction was studied. The results show that the microstructure of the sample with dimension of ϕ8 mm×25 mm was composed of acicular α clusters at prior β columnar grain boundaries and net-basket-shaped widmanstatten structures formed by interlaced acicular α phase in grains. The axis of prior β columnar grains was parallel to the direction of powder deposition and grew through every deposition layer. The residual β phase dispersed among α phase matrix. The top microstructure of the sample with dimension of ϕ25 mm×8 mm consisted of acicular α' phases. The acicular α' phase in the middle microstructure was decomposed into α phase and β phase, and the width of α phase was relatively large. The strip α phase in the bottom microstucture grew through prior β grains to form clusters. The residual β phase dispersed in the continuous network among α phase matrix. The microhardness of the part with acicular α or α' phase of large length-width ratios was higher than that with strip and lamellar α or α' phase of small length-width ratios.