Oxidation kinetics and the effect of oxide scale on interfacial microstructure of polycrystalline aluminum nitride substrate metallized with conductor and resistor thick-film pastes have been investigated over an oxidation temperature range of 1150–1450 °C at 50 °C intervals in ambient air. At temperatures below 1150 °C, weight gain increased linearly with the isothermal time. The grown oxide layer was porous and was not protective at the temperature, revealing an interface-controlled mechanism. By contrast, the oxidation kinetics exhibited a parabolic behavior as the temperature was raised further above 1250 °C, which is indicative of a diffusion-controlled mechanism. The oxide scale effectively reduced the population of gas bubbles located at the film-substrate interface when coated with the resistor film on the AlN substrate, while the conductor paste provided a hermetic interface even when the AlN was not pre-oxidized. Elemental analyses showed that glass composition of the pastes, e.g., lead, was crucial in determining the interface defects. The differences in interface microstructure of the resistor and conductor metallization, as well as the implication of the thick-film structures to the formation of blisters at the interface, are discussed.