Colloidal ceria slurry is expected to be a candidate of alternative slurry for oxide film CMP because the slurry particles are small and have a regular shape comparing to conventional calcined ceria particles. Due to the small diameter of the particles, removal rate(RR) tends to be smaller than that of calcined ceria slurry. To improve the RR, KOH additive was mixed to the slurry and the influence of KOH concentration was investigated. The result showing RR transition and dramatic improvement of RR close to that of calcined ceria slurry has been obtained. Even in the high RR, surface roughness of the substrate was kept almost constant in good level. To investigate the mechanism of the RR improvement, SEM images of particles in different states were captured. Original colloidal ceria particles have a polyhedral shape close to spherical one. With KOH additive, particles aggregated but the strength of aggregation was apparently different in different KOH concentrations. The aggregation was quite strong over 0.1mol/L concentration of KOH, and the RR improvement strongly relates to the aggregation state. Although the aggregated particle size was comparable to the calcined ceria particle size, the surface roughness was better than that of calcined ceria slurry.