Optical fiber sensors based on light backscattered by particles are widely used to measure the size and/or the concentration of suspended particles. They are used in several applications like Chemical Mechanical polishing (CMP). An optical device for high Concentrated CMP silica slurry monitoring is presented. To predict the response of the probe, a Monte Carlo simulation was developed. This code was validated by using calibrated latex beads. A comparison between calculation and experimental data is given for dense suspensions of silica particles at different sizes and concentrations. A good correlation between measurement and simulation was found.