Disclosed is a composition for the separation of a serum or plasma, which comprises a cyclopentadiene oligomer, can be used for separating a serum or plasma by utilizing the difference in specific gravity among blood components, and enables to provide a serum or plasma having no oily component floating therein after centrifugal separation. The composition comprises a trimellitic acid ester and/or a pyromellitate ester and a cyclopentadiene oligomer. Alternatively, the composition comprises an aromatic esterified compound and a cyclopentadiene oligomer as the main ingredients and additionally comprises an inorganic micropowder and an organic gelling agent as thixotropy-imparting agents, wherein the organic gelling agent is contained in an amount of less than 0.06 parts by weight relative to 100 parts by weight of the cyclopentadiene oligomer.
