The capability of atomic layer deposition (ALD) to coat conformally complex 3D nanotopography has been examined by depositing amorphous, polycrystalline, and single crystal TiO2 films over SnO2 nanowires (NWs). Structural characterizations reveal a strong correlation between the surface morphology and the microstructures of ALD films. Conformal growth can only be rigorously achieved in amorphous phase with circular sectors developed at sharp asperities. Morphology evolution convincingly demonstrates the principle of ALD, i.e, sequential and self-limiting surface reactions result in smooth and conformal films. Orientation dependent growth and surface reconstruction generally lead to nonconformal coating in polycrystalline and single-crystal films. Especially, an octagonal single-crystal TiO2 shell was derived from a rectangular SnO2 NW core, which was the consequence of both self-limited growth kinetics and surface reconstruction. Models were proposed to explain the conformality of ALD deposition over 3D nanostructures by taking account of the underlying microstructures. Besides the surface morphologies, the microstructures also have significant consequence to the surface electronic states, characterized by the broad band photoluminescence. The comparison study suggests that ALD process is determined by the interplay of both thermodynamic and kinetic factors