The aim of this study is to clarify the metal-mold reaction for the economic net-shape forming of titanium and titanium alloys. The metal-mold reactions between pure titanium, TiAl alloys and Al2O3 mold were examined in a plasma arc melting furnace. The alpha-case generation between pure titanium and Al2O3 mold could not be explained by the conventional α-case formation mechanism, which is known to be formed by the interstitials, especially oxygen dissolved from mold materials. However, based on the interstitial and substitutional α-case formation mechanism, the α-case formation between pure titanium and Al2O3 mold, and α-case free casting of TiAl alloys against Al2O3 mold can be explained. On the basis of the interstitial and substitutional α-case formation mechanism, α-case controlled net-shape forming of titanium and TiAl alloys can be possible for the structural materials.