In this study the effects of oversize particle contamination in chemical mechanical polishing (CMP) slurries were investigated on the silica CMP process. The limits of light scattering technique were established in detecting coarse particles in a commercial silica CMP slurry using two different methods. The detection limits were set by observing the shift in particle size distribution curve or by the appearance of an additional peak in the particle size distribution curve of the baseline slurry when a known amount of coarser particles were added to it. Simultaneously, polishing tests were conducted by spiking the base slurry with coarser sol-gel silica particles at the established detection limits. It was observed that the contamination of larger particles not only created surface damage but also changed the material removal rate. The mechanism of polishing in the presence of larger size particles is discussed as a function of particle size and concentration.