Petal-like alumina flakes were prepared by hydrothermal reaction at 170°C for 6 h with aqua-based precursor materials. The gel particles were calcined at 550°–1200°C. The gel and calcined particles were characterized by differential thermal analysis (DTA), thermogravimetry (TG), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), multipoint Brunauer–Emmett–Teller (BET) surface area, and field emission scanning electron microscopy (FESEM). Crystallization of alumina was confirmed by DTA and XRD studies. XRD results showed that boehmite particles were formed in as-prepared sample while γ-alumina (γ-Al2O3) persisted up to 800°C followed by the appearance of θ-alumina (θ-Al2O3) at 1000°–1200°C. The characteristics Al–O vibrations in alumina polymorphs were examined by FTIR study. FESEM confirmed the formation of petal-like alumina flakes with the assembly of alumina nanorods.