The reduced NiFe2O4 has high activity of CO2 decomposition and can be used repeatedly. The performance of NiFe2O4-doped 4wt% Cr3+ (NiCr0.08Fe1.92O4) is much better than non-doped sample. The fresh NiCr0.08Fe1.92O4 is of spinel structure and nanometric scale. During H-2/CO2 cycle reactions their spinel structure is gradually destroyed and new species such as carbon, FexNi1-x (0 <= x <= 1) alloy and Fe3C appear. Their morphology, phase composition and pore structure are characterized by means of X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and low-temperature N-2 adsorption. The crystallite size increases with number of reaction cycle. From TEM pictures, it is found that deposited carbon on the surface is amorphous at first and transformed gradually to crystallite carbon, even in carbon nanotubes which increase with number of cycle reaction. (c) 2007 Elsevier B.V. All rights reserved.