A polishing composition includes: metal oxide particles as abrasive grains, wherein the full width at half maximum of a peak portion having the maximum diffracted intensity in an X-ray powder diffraction pattern of the metal oxide particles is less than 1°; and two or more varieties of water-soluble polymers having different weight-average molecular weights as a selectivity control agent, wherein the water-soluble polymers differ in weight-average molecular weight by 10 times or more. A polishing composition and a method for polishing a semiconductor substrate using thereof can suppress occurrence of defects due to polishing such as a scratch, dishing, and erosion while keeping the high polishing rate, and can optionally control the selectivity, which is a ratio of polishing speed of a metal layer and that of an insulator layer.