A series hematite structure iron oxide nanoparticles of various sizes was synthesized and tested in the soot combustion in oxygen rich conditions in the tight and loose contact modes. The catalysts were characterized by XRD, TEM, and Raman Spectroscopy, whereas the catalytic reactivity with model soot (Printex U) was evaluated by TGA/DTA method. It was found that the catalysts activity is size dependent for the particles dimensions in the range of 5-100 nm with the smallest one exhibiting the highest activity (lowest soot ignition temperature). The most active nanohematite particles, formed in situ in the laboratory and real process conditions of light fuel oil combustion with addition of fuel borne catalyst (FBC) precursors, exhibit the beneficial size range of 5-15 nm. (C) 2015 Elsevier B.V. All rights reserved.