Communication: Supported type cocatalysts using triphenylcarbenium perchlorate (Ph3CClO4) were prepared by impregnation on inorganic carrier, magnesium chloride (MgCl2) and applied to ethylene polymerizations with rac-Et[Ind](2)ZrCl2. Homogeneous polymerizations with Ph3CClO4 were also carried out for comparison. The activity of homogeneous polymerization was much lower than that obtained with methylaluminoxane (MAO). On the other hand, rac-Et[Ind](2)ZrCl2 activated by the supported type Ph3CClO4/MgCl2 system displayed high activity comparable to that obtained with MAO. From the results of fractionation and polymerization of the rac-Et[Ind](2)ZrCl2-Ph3CClO4/MgCl2 catalyst system, it was found that the increased activity mainly came from the active species in the supernatant part. UV-vis spectroscopic measurements combined with ICP analysis indicate that the active species in the supernatant fraction are composed of a stoichiometric amount of perchlorate and metallocene catalyst.