Ordos basin has favorable condition for the research work. On the basis of various sorts of data, 8 parasequences were identified in Upper Ma 5 Member of the basin. Within each parasequence, content of substances indissoluble in acid, content of remnant gypsum,δ13C values and NG logging curve display regular changes, indicating the controlling role of sea_level fluctuation. These parameters can provide new methods for the division of paresequence boundaries. Among differnet diageneses, dissolution in the upper part of each parasequence is stronger than that in the lower part. In most parasequences, dolomite in the lower part seems coarser than that in the upper part, and within most beddings pressure solution also occurs more likey at the bottom than in the upper part. It is discovered that the average porosity of exposed parasequence boundaries is similar to that at sequence boundaries, suggesting the larger contribution of parasequences boundary to the preservation of porosity than sequence boundary. Hence the exposed parasequence boundaries deserve more attention. If overlapped, a set of exposed parasequences boundaries would definitely become wonderful reservoirs for oil and gas.