The present paper reports experimental results relative to the on-line characterization of particle size during a wet grinding process-thanks to an optical analyser (Turbiscan On-line, FORMULACTION). During batch grinding processes, the optical system, based on the multiple light scattering method, is placed in a derivative loop and the light flux is analysed to characterize the kinetics of fragmentation. In the case of continuous processes, the optical analyser placed on-line at the outlet of the mill allows to determine a characteristic size of the fragments without preliminary dilution. The influence of various operating parameters (solid volume fraction, feed flow rate) on the sensibility of the answer of the system is discussed. Moreover, the sensor allows determining the delay time to reach a steady state when starting the process or furthering to a disturbance.