Some SiO2-supported neodymocene catalysts were prepared and polymerization of ethylene was conducted with them using alkylaluminiums, methylaluminoxane (MAO), BuLi and Bu MgEt (Bu: butyl, Et: ethyl) as cocatalysts. The lifetime of these catalysts was found to be very long. Catalytic activity as well as molecular weight of the produced polymer are strongly dependent upon kind and amount of cocatalyst used. With increasing polymerization temperature, the activity increases monotonously up to 80 degrees C. The use of BuMgEt and Al(i-C4H9)(3) as cocatalyst gives the highest activity and highest molecular weight, respectively.