The ingot of Ti-6Al-4V alloy was prepared by investment casting. Microstructure and formation mechanism of alpha-case in the surface layer of the ingot were investigated by optical microscope (OM), electron probe micro-analyzer (EPMA) and Vickers hardness. The results show that the alpha-case covers the whole surface of the sample, and the microstructure of the alpha-case features in a coarse plate-shaped structure which ranked as-basketweave. The formation mechanism of alpha-case is associated with the diffusion of interstitial oxygen and carbon atoms in the shell material toward matrix alloy. Depth of alpha-case obtained from micro-hardness testing is slightly deeper than that observed by OM. More reliable results come from the micro-hardness testing for removing the alpha-case completely.