A qualitative study on the effects of humidity on activity of aluminum nanoparticles has been reported in this letter. The Al nanoparticles were synthesized by the process of hybrid induction and laser heating (HILH). In order to understand the stability of the oxide coating from aluminum nanoparticles, the Al nanoparticles were examined using high-resolution transmission electron microscopy (HRTEM) and the thermal behaviors were identified by DSC-TG. It was found that a compact amorphous oxide layer of 3 nm in thickness covered the original Al nanoparticles. After stored for 8 weeks, the oxide layer grew up to 5 nm in thickness, and the oxidation diffused to the interior of Al nanoparticles. The results indicate that the activity of Al nanoparticles is deeply influenced by the environment, especial the humidity. A higher relative humidity would accelerate the aging of Al nanoparticles. The DSC-TG results show that the oxidation of Al nanoparticles occurs at least in two steps.