Abstract: Olistostrome of gravity flow are found in the Middle and Upper Ordovician in the western and southern margin of Ordos Basin, Some of great boulders, and allochthons as irregular long-cylinder or stratoid structure with ten to one hundres meters long are found in parts of the strata, as the allochthons are dishar-monious obviously with the host rock, it suggests that the orgins are different. The pefrdogic characters, stratigraphic horizon, the assembledge of rock and forming of olistostrome ard discussed. It proposed by the author that the olistostrome is formed on the steep gradient zones of passive continental margin of rifts, where great boulders and allochthons are produced by collapse and carried by gravity flows.