The present invention relates to a modifier useful for modifying a polymer, a method for preparing a modified and conjugated diene-based polymer using the same, and a modified and conjugated diene-based polymer prepared thereby. In the modified and conjugated diene-based polymer, a hydroxyl group and a silyl group are combined, and the affinity thereof with a silica-based filler may be excellent. Accordingly, a rubber composition including the modified and conjugated diene-based polymer may have excellent processability, and as a result, molded articles (for example, tires) manufactured from the rubber composition may have excellent tensile strength, abrasion resistance, and viscoelasticity properties.