The exploration towards cost-effective filler metal for ceramics joining has always been the key issues for ceramics joining. Herein, we reveal that the Al metal prefers to spread on the ZrO₂ based ceramic under the air heating condition, due to the geometric limit effects by in-situ formed dense Al₂O₃ surface. Inspired by this, the joining of ZrO₂ based ceramics was realized in air with Al metal as filler, through the diffusion of Al towards ceramic side. The Al element can induce obvious interfacial bonding effect on Al₂O₃ layer and ZrO₂ ceramic, where the hybridization among the Al-p, Zr-d and O-p orbitals plays a key role. The in-situ formed Al₂O₃ layer on Al filler surface is vital for forming the fine interface (shear strength of ~36 MPa), which results in the relief of lattice mismatch and peak stress at ceramic-filler metal transition interface.