Nanocrystallites of six mixed ternary transition metal ferrite (MTTMF) were prepared by co-precipitation method and characterized by X-ray diffraction (XRD) and BET equation. XRD patterns gave average particle size for NiZnFe2O4 (NZF), CuCoFe2O4 (CuCoF), NiCuFe2O4 (NCuF), CuZnFe2O4 (CuZF), CoNiFe2O4 (CoNF) and CoZnFe2O4 (CoZF) as 7.2, 3.0, 3.0, 6.8, 3.9 and 5.8 nm, respectively. Further, catalytic activities of the MTTMF nanocrystallites on thermal decomposition of ammonium perchlorate (AP) were investigated using thermogravimetric analysis (TG), differential scanning calorimetry (DSC), differential thermal analysis (DTA), and ignition delay measurements. IR-aciive internal mode of cation and anion of ammonium perchlorate changes with temperature in presence of MTTMF nanocrystallites. The catalytic activity was found to be in the order: CoZF > CoNF > CuZF > CuCoF > NCuF > NZF. Nanocrystals of MTTMF have been proposed to play a significant role in the decomposition of AP (C) 2008 Elsevier B.V. All rights reserved.