Three different aluminum nanopowders with mean particle size of 20 nm,25 nm and 50 nm were synthesized by high frequency induction evaporation condensation method. Structure and morphology of these aluminum nanopowders were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM),and the thermal behaviors of them were studied using differential thermal analysis (DTA). The results show that the enthalpy change for oxidizing reaction of Al nanopowders is reduced from 3.721 kJ·g^－１ to 0.928 kJ·g^－１ as Al particle size decreases from 50 nm to 20 nm，so the reactivity of aluminum nanopowders becomes low as the particle size decreases，which is possibly due to the increase of surface oxide of aluminum nanoparticles and the absence of stored energy in aluminum nanopowders. Therefore,the reactivity of Al nanopowders in air depends on the particle size,the preparation method,as well as the type,structure and thickness of the surface passivating layer.