The effect of elevated pressures on the soot formation in coaxial ethylene-diffusion flame was investigated under different ventilation conditions, i.e. one with a chamber valve closed and the other with a chamber valve open. Experimental measurements include the soot volume fraction and macroscopic flame behaviors such as visible height and flame shape. The presence of ventilation-assisted flow around the flame was found to further increase the visible flame radius and height at elevated pressures. The maximum soot volume fraction measured within the flame was also increased with the ventilation assisted flow as well as with the ambient pressure. Experimental results clearly indicate that changes in the physical appearance of the flames altered by ventilation-assisted flow are an important factor to affect the sooting behavior in coaxial diffusion flames. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.