Three different size aluminum nanoparticles are introduced in order to study the effect of particle size on reactivity and combustion characteristics. Structure and morphology of these aluminum nanoparticles are characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that the mean particle sizes of these aluminum particles are 60 nm, 80 nm, and 110 nm with amorphous surface oxide. Thermogravimetry and differential thermal analysis (TG-DTA) and the burning performance (burning rate and pressure exponent) measurement are used to characterize the reactivity of three kinds of aluminum nanoparticles. It is found that the reactivity of aluminum nanoparticles is increased at first and then decreased as the particle size decreasing from 110 nm to 60 nm, which is possibly due to double function of the high specific surface area. Therefore, the reactivity of aluminum nanoparticles depends on many factors, it is important to consider all these factors before pursuing the small particles.