Small-angle X-ray scattering (SAXS) technique has been used to study soot formation and growth in an ethylene flame doped with Ni(acac)(2) (nickel acetylacetonate) at atmospheric pressure. An undoped flame under the same conditions was also studied to compare with the doped flame. The results show that the Ni(acac)(2) addition affects particle size, shape and morphology significantly at certain flame heights. It has reducing effect on the primary soot particle size at the flame heights between 17 and 34mm. It delays the growth of primary soot particles at soot growth region. The soot volume fraction in the doped flame is smaller than that in the undoped flame by factors of 1.5-4 at different heights above the burner. In the most heights of the soot surface growth region (HAB = 13-30 mm), the Porod exponents for the doped flame are smaller than those in the undoped flame, suggesting that the addition of Ni(acac)(2) in the ethylene flame induces a rougher surface and more irregular shape of the soot primary particles. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.