is 15 nm an average particle diameter d 2 of the second colloidal silica is 75 nm and the ratio in difference of particle diameters d 2 d 1 is 5 the primary particle diameter of the colloidal silica can be determined through observation by an sem or tem for example after being diluted slurry is uniformly deposited on a sample table and heated to evaporate the liquid component to form a layer thereafter gold for example is vapor deposited on the layer and the resultant layer is observed by an sem and photographed at a magnification of 100 000 to 500 000 times thereafter the maximum particle diameter of the particle is measured by using calipers then a diameter is determined by drawing a perpendicular bisector relative to the maximum particle diameter and an added average of values obtained from the diameter thus determined is assumed as the primary particle diameter after 100 values in number of the primary particle diameter have been determined these values are employed to draw a cumulative grain size curve from which a primary particle diameter at 50 is assumed as an average particle diameter the cmp slurry according to the embodiments of the present invention can be prepared by dispersing the abrasive grain containing the first colloidal particle and the second colloidal particle in water such as pure water the abrasive grain containing the first colloidal particle and the second colloidal particle should preferably be included in the slurry at a ratio of 0 1 wt to 20 wt if the content of.