Nano-scale mesoporous alumina powder (γ-Al₂O₃) was synthesized by a precipitation-ultrasonic treatment-calcination method using aluminum nitrate and ammonia solution as raw materials and ethyl diamine (EDA)/polyethyleneglycol (PEG) as co-template agents. The samples were characterized by transmission electron microscope (TEM), X-ray diffraction (XRD), N₂ adsorption-desorption and acid-base titration methods. The results showed that the samples can be transformed into nano-sized mesoporous γ-Al₂O₃ with needle or rodlike morphology, centralized pore size distribution and stable structure. The average pore size, pore volume and specific surface area of the obtained sample were 5.8 nm, 1.13 cm³•g^－1 and 510 m²•g^－1 respectively. A co-templating mechanism of EDA and PEG in the synthesis of mesoporous alumina was discussed based on the chemical reactions and speciation of EDA and alumina surfaces as a function of pH in aqueous solution.