The aggregation behavior of ion-exchanged nickel species ranging from 0.77 to 14.6 wt % in loading on Loy Yang brown coal was studied. The exchanged nickel species aggregated to metal particles at 600-770 K during pyrolysis, depending on the amount of exchanged nickel species. The evolution of volatile components was affected by the amount of exchanged nickel species. Two peaks of CO evolution appeared, one at 650-770 K, depending on the amount of exchanged nickel species and the other at ca. 880 K, comparable to a broader peak at 870 K for an acid washed coal. The CO evolution peaks become larger with increasing nickel loading. The CO2 evolution peak appeared at 650 K and it became larger with increasing nickel loading, and a new CO2 evolution peak appeared at 850 K for the nickel loadings more than 4.2 wt %, which was negligible for the acid-washed coal. The decomposition behavior of Loy Yang coal is explained by the aggregation behavior of nickel species.