In this work, the effective density of soot particles was studied in burner-stabilized-stagnation premixed ethylene flames, at an equivalent ratio of 2.0 and over the maximum flame temperature of 1747 K < T-max < 1837K, using micro-orifice probe sampling in tandem with a centrifugal particle mass analyzer, a differential mobility analyzer (DMA) and a condensation particle counter. For a better understanding of the variation of effective density with mobility diameter, monodisperse soot particles between 10 and 50 nm were classified via the DMA and imaged by transmission electron microscopy (TEM). The particle size distribution functions and the C/H ratio of the particles were measured by a scanning mobility particle sizer and an elemental analyzer, separately. The results show that for all these flames, effective density first increases and then decreases with increasing mobility diameter, and the maximum occurs at the mobility diameter of similar to 20 nm, which is consistent with the size for soot morphology changing from spherical-like to non-spherical shapes according to the TEM images. With the increase of flame temperature, the effective density decreases, which is caused by the slower particle growth rate at higher temperatures as shown by a series of evidences. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.