Abstract: A new particle monitoring technique of light extinction mode is introduced, of which the principle is based on the fact that when a light beam transmits the flowing suspensions containing suspending particles, different fluctuation degree of transmitted light intensity can indicate the aggregation state of particles. Both the theoretical calculation and practical applications show that this on-line particle monitoring method can reflect the aggregation state and the relative size change of particles. The floc formation process and the mixing conditions can be shown in detail in monitoring the coagulation process of water treatment. At the same time, the detailed process of disintegration or dispersion of granular sample under the action of shearing condition can also be seen. Therefore, the technique can be used as an effective means and method to study the aggregation and disintegration process of particulate matters.