The experimental setup consists of a coannular laminar diffusion flame burner and a laser light scattering/extinction system. The light source in the experiment is a optically pumped semiconductor laser operated at 639 nm and 0.5W. A mechanical chopper and a lock-in amplifier are used to improve the signal-to-noise ratio. The light transmitted through the flame is detected by a photodiode for the light extinction measurement. And the laser light attenuation is related to the soot volume fraction through Beer’s law. The scattered light is detected by two photo-multipier tubes at 45° and 135° to the incident beam for the light scattering measurement. And the intensity of the light scattered by the soot particles in the flame is related to the soot properties (primary particle diameter, the number of primary particles per aggregate, the radius of the gyration of the aggregate) through the Rayleigh-Debye-Gans (RDG) fractal aggregate theory.