Dynamic light scattering (DLS) based on a monodisperse model was applied to ultrafine particle formation in a flame where particles grow and deform within several ms, with occasional high number density and broad size distribution. DLS was experimentally evaluated as an in-situ technique of measuring mean particle size by comparing with TEM (transmission electron microscope) photographs. The following results were obtained. 1. For particles formed in a flame, a mean particle size of several tens of nanometers by DLS with monodisperse assumption was in good agreement with the volume mean diameter obtained from TEM photographs, except for particles strongly deviated to small size. 2. For particles with a size distribution strongly deviated to the fine side, DLS with monodisperse assumption gives a considerably large value, around twice the volume mean diameter. This value still reflects a plausible mean size of large particles contained in the measuring volume.