The exceptional chemical inertness and superior high-temperature wear resistance of chemical vapor deposition (CVD) Al 2O 3 coatings make them indispensable for a range of demanding cutting applications. The performance of these coatings is determined by their phase and crystallographic orientation, where phase control serves as the fundamental basis for orientation control. This study addresses a central question: how does the design of the nucleation surface (intermediate layer) control the phase composition and interfacial structure of CVD-Al 2O 3? We systematically investigated three tailored intermediate layers—TiC, Ti (C 0.5,O 0.5), and Ti 2O 3—to decipher the mechanisms of Al 2O 3 phase evolution based on lattice matching and the correlated interfacial energy. High-resolution transmission electron microscopy revealed that TiC intermediate layer promotes the growth of a metastable κ-Al 2O 3 phase, with an orientation relationship determined as (001) κ-Al2O3 // (111) TiC. Nucleation of κ-Al 2O 3 on the fcc-Ti (C 0.5,O 0.5) intermediate layer occurs via an ultrathin γ-Al 2O 3 transition region, with an orientation relationship (001) κ-Al2O3 // (111) γ-Al2O3 // (111) Ti(C0.5,O0.5). Notably, the trigonal Ti 2O 3 layer enables the direct growth of a stable α-Al 2O 3 phase, following an orientation relationship (104) α-Al2O3 // (104) Ti2O3. Density functional theory calculations corroborate these findings, showing high adhesion work for these interfaces (3.97, 6.91, and 2.07 J/m 2, respectively), confirming their exceptional stability. This work establishes intermediate layer engineering as a powerful strategy for the precise phase and microstructure control of CVD-Al 2O 3 coatings.