Uniform alumina microfibers precursor (ammonium aluminum carbonate hydroxide, AACH) were successfully synthesized using the microwave hydrothermal method, with average length of 5–10 μm and the diameter around 300–500 nm. FT-IR spectra indicated AACH was composed of NH4[Al(OOH) HCO3]·H2O. The thermal behaviors of the as-prepared AACH were investigated using differential scanning calorimeter and thermogravimetric analysis (DSC-TG), XRD, and SEM. The results showed that the thermal decomposition of the AACH microfibers occurred via three steps, which were respectively divided into adsorption of physical water, dehydration of crystalline water, and decomposition of AACH. The activation energies for the above three steps were calculated using Coats-Redfern method. The phase transformation sequence was found to be AACH→ amorphous Al2O3→ γ- and θ-Al2O3→ α-Al2O3. It was also observed that the thermal treatment had little influence on fiber morphology of the products. The fibers morphology with high thermal stability will endow to prepare alumina microfibers with novel application potentials.