The invention relates to a supported metallocene catalyst composition and a process for the preparation of polyolefin using the same. A supported metallocene catalyst composition according to the invention is prepared by bringing a compound of a group IV transition metal into contact with an inorganic or organic porous carrier treated with an ionic compound. Advantages of a supported metallocene catalyst composition of the invention include an increase in the catalyst activity during polymerization of slurry and an olefin compound in the vapor phase even at a low content of metallocene metal components within the carrier, and an improvement in solving process problems such as fouling, sheeting, plugging or the like.