Interfacial reactions between zirconia and titanium were proceeded by the infiltration of liquid titanium through the open pores of zirconia. During reaction, zirconia was reduced to oxygen-deficient zirconia (ZrO2-x) and resulted in the liberation of oxygen. The dissolution of oxygen in titanium gave rise to the needle-like α-phase as well as TiO2 in the reaction zone. The content of oxygen in ZrO2-x increased because of the ex-solution of proeutectoid spherical particles α-Zr(O). On cooling, secondary ZrO2-x crystallites with O/Zr ≈ 1.9 were found. Furthermore, the Ca content increased in the solid solution of ZrO2-x that could induce the precipitation of CaZrO3.