The interfacial reaction between (Pr _0.8 Sr _0.2 ) _y MnO ₃ (PSM, y=0.9, 1.0, 1.05) film and 3 mol% yttria tetragonal zirconia (TZ3Y) substrate has been studied at 1200 and 1400°C in air. A diffusion layer of Pr and Mn in zirconia, which was identified to be a cubic phase of zirconia, was detected in all specimens. When the solubility limit of Pr ions in the cubic zirconia was reached, a pyrochlore phase, Pr ₂ Zr ₂ O ₇ , was formed. A delay for the formation of pyrochlore phase was observed for the A-site sub-stoichiometric and stoichiometric PSM at 1200°C. For the A-site over-stoichiometric PSM, a Pr-rich (Pr,Zr)O _x phase was detected at the interface besides the pyrochlore phase. At 1400°C, a relatively thick layer of pyrochlore phase was formed after 24 hour heat treatment in all specimens. The amount of the pyrochlore phase formed at the interface depends on the A-site stoichiometry of perovskite in the initial stage. The growth of the pyrochlore layer after the initial stage, however, appears to be determined by contact area between PSM and the substrate.