A colloidal silica slurry has been manufactured by mixing nano silica powders having different grain size to improve the reliability of Sapphire wafer CMP process. The main reliability problem of CMP process such as the breaking of wafer can be prevented by reducing the size of particles in a slurry. While existing commercial colloidal silica slurries are usually made of single grain size silica powder of about 120nm, in the present study 40nm and 100nm silica powders are mixed to achieve a similar removal rate. The new colloidal silica slurry showed wafer removal rate of $3.04{mu}m/120min$ while that of a commercial colloidal silica slurry was $3.03{mu}m/120min$. The roughness was less than $4{AA}$ and scratch was 0. It is also expected that the reduction of the size of nano silica particles can improve the dispersion stability and prolong the useful life of the slurry.