The analysis of thin sections and cathode luminescence for core samples has shown several petrological characteristics of Ordovician hydrothermal activities in the central and western parts of Ordos Basin. The hydrothermal products commonly are hydrothermally-origined tuff, saddle dolomite, mottled pyrite and authigenic quartz and some hydrothermal mineral assemblages including quartz-fluorite and lapis lazuli-calcite, in which the hydrothermally-origined dolomite shows dark color in cathode luminescence. It is discovered that the strike of the hydrothermally active field follows the trend of faults in the western margin of the basin and the average porosity of hydrothermally-influenced reservoirs is 2,~3, higher than that of general reservoirs. It is concluded that hydrothermal activities play a constructive role in the Ordovician hydrothermal reservoirs, which are expected to be one of the most favorable exploration targets.