The present invention relates to a CMP slurry composition comprising an abrasive particle; a dispersant; an ionic polymer additive; and a non-ionic polymer additive including a polyolefin-polyethylene glycol copolymer including at least two polyethylene glycol repeat unit as a backbone and at least a polyethylene glycol repeating unit as a side chain, and a polishing method with using the slurry composition. The CMP slurry composition shows a low polishing rate to a single-crystalline silicon layer or a polysilicon layer and a high polishing rate to a silicon oxide layer, resulting in having an excellent polishing selectivity.