Study to get a high activity for olefin polymerization by a supporting Ziegler-type catalyst on MgCl2 was carried out. Only heating anhydrous MgCl2 with TiCl4, very little amount of TiCl4 was fixed because of high crystallinity and small surface of anhydrous MgCl2. A pretreatment by some organic electron donner such as butanol or methyl acetate before the reaction with TiCl4, resulted a considerable fixation of TiCl4 onto MgCl2 and gave high activity for ethylene polymerization by 70—90 times of conventional TiCl3 catalyst. The study of the TiCl4–MgCl2 comilling, the other supporting technique, was also investigated. There are efficient Ti and inefficient ones to participate in the formation of active species among the fixed Ti. The titanium fixed in the initial stage of milling at the active site of broken MgCl2 particle surface was rather efficient. Furthermore, The high stereospecificity was performed for propylene polymerization, by arranging some organic electron donner around Ti for the above mentioned TiCl4–MgCl2 catalyst.