The effects of four different types of investment on the alpha-case layer of Ti6Al7Nb (Ti67) castings are summarized. The formation mechanism of the alpha-case layer is investigated. Four different investment materials are compared. The alpha-case layers on castings produced with Invest-TiT, Al2O3, and ZrSiO4-based investments comprise an oxide layer, an alloy layer, and a hardening layer, and are not only formed by interstitial oxygen but also by substitutional elements dissolved from mould materials. The Y2O3-based investment material provides the most-reliable choice for the production of titanium castings and is beneficial for the casting of Ti67 because of no alpha-case formation.