Aluminum nitride (AlN) is highly insulating and has a high thermal conductivity. AlN also has the advantages of being nontoxic and chemically stable. Therefore, it is a suitable sealing material for electric devices. Previous studies have shown that the addition of NHCl has a large influence on the formation of AlN and can effectively be used in its low-temperature synthesis without the use of special equipment. However, it has not been clarified whether NHCl simply promotes the reaction between Al and nitrogen or directly contributes to the nitridation reaction. In this study, which was part of a series of studies on the development of low-temperature synthesis methods for AlN, the nitridation behaviors of Al in Al-N, Al-NHCl-N, and Al-NHCl-He systems were determined, and the effects of the heating temperature and amount of NHCl on the nitridation behavior were examined in detail. When NHCl was added, AlN began to form at 600 °C, a formation temperature that was approximately 200 °C lower than that when only Al powder was heated under a nitrogen stream. The fact that the formation of AlN was also observed when the NHCl-added Al powder was heated under a helium gas stream confirmed that nitrogen derived from NHCl contributed to the formation of the AlN. Furthermore, based on the experimental results, the reaction mechanism was clarified, and the kinetic parameters for the nitridation of Al were determined.