Abstract: On-line monitoring of self-flocculating yeast cell particles (SFYCPs) that were characterized by their weak strength, irregular morphology, special size distribution, and so on, was realized by using the Focused Beam Reflectance Measurement (FBRM) technique.The experimental results showed good correlation between the total counts of the FBRM chord length and the biomass concentration of SFYCPs, which could be used to measure the biomass concentration for the SFYCP system quantitatively.Furthermore, the mode of FBRM chord length distribution was linearly correlated to both the mean (R²=0.986) and the 95th percentile (R²=0.988)of the length-weighted chord length distribution, which could be used to evaluate SFYCP size distribution effectively.The CO₂ bubbles produced during cell growth/fermentation were observed and proved to create disturbance on on-line measurement of the SFYCP system.This disturbance could be excluded by estimating SFYCP size distribution based on the mode of FBRM chord length distribution.