Soot, which is produced through the rapid growth of hydrocarbon molecules in fuel-rich flames, has been proved to be responsible for global warming and harmful to the respiratory system of human. However, the complex formation process of soot in flames has not been understood yet because of the lack of efficient time-resolved methods for monitoring soot and its precursor species. We present here a viable pump-probe approach to measure ultrafast processes of the soot particles in flames with femtosecond time resolution. The scattering of femtosecond ultraviolet (UV) light pulses is used to monitor soot particles along alkanol-air diffusion flames from the inception phase up to the burnout region after the formation of a femtosecond laser filament in the flames. The time evolution of the scattered UV signal reveals unexpected ultrafast swelling and shrinking of soot particles induced by the femtosecond laser filament in the different flame regions. Our in situ and time-domain measurements of soot particles with the femtosecond time resolution unveil their unique properties in the flames. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.