Aluminum-doped lithium lanthanum zirconate (Li_6.25Al_0.25La₃Zr₂O₁₂, Al-LLZ) has a relatively high Li⁺-ion conductivity of ~10^-4 S cm^-1 at room temperature and a wide electrochemical window enabling to use both lithium-metal anode and high potential cathodes, so that it has been focused as a promising solid electrolyte for all-solid-state rechargeable lithium batteries with high energy density. Several kinds of fabrication methods, which usually include heat treatment processes, have been utilized to fabricate cathode on Al-LLZ. However, the effect of heat treatments on the formation of impurities at the interface between Al-LLZ and cathode has not been fully understood yet. In this study, we investigated the thermal compatibility of conventional cathode materials (LiCoO₂, LiMn₂O₄ and LiFePO₄) with Al-LLZ by X-ray diffraction (XRD) and charge-discharge tests. The XRD analysis showed that LiCoO₂/Al-LLZ mixture is stable up to 800°C, but impurities are formed in LiMn₂O₄/Al-LLZ mixture and LiFePO₄/Al-LLZ mixture at 600 °C and 400 °C, respectively. This result indicates that the limit of heat treatment temperature for the formation of electrode/Al-LLZ interface strongly depends on the kind of electrode materials.