Abstract: A new application of dynamic light scattering (DLS) is disclosed for the use in accurately and reliably determining the size and/or size distribution of microscopic particles in laminar or turbulent flow. A system of microscopic particles in a fluid medium are forced to be in a laminar, rotational and/or turbulent flow. Then, a coherent light such as laser light is launched into the flowing particles, and the scattered light from the flowing particles is collected, converted into electrical signal, and processed to numerically derive the size/size distribution of the particles in the flow. The technique of DLS has long been used to determine the size/size distribution of the particles in dispersion under static condition. And, it has long been thought that accurate and reliable size determination of the particles in flow is not feasible. Therefore, present invention is the first ever to disclose that the technique of DLS can be used to accurately and reliably measure the size of the particles in flow.