A composition for blood serum or plasma separation which has improved compatibility between a cyclopentadiene oligomer and a phthalic ester and generates no oily ingredient suspending in the blood serum or plasma after centrifugal separation regardless of the conditions for the centrifugal separation; and a vessel for blood examination which contains the composition. The composition for blood serum or plasma separation comprises: a polycyclic hydrocarbon compound having an unsaturated and/or saturated cyclic structure in the molecule and having a solidifying point or pour point of 0.degree. C. or lower; a cyclopentadiene oligomer; and a phthalic ester.