The blood compatibilities have been studied for block copolymers composed of poly (dimethylsiloxane) segments linked to polypeptide which were synthesized by reacting primary amine-terminated silicone with r-benzyl-L-gultamate N-carboxy anhydrides, and also studied for copoly(α-amino acid)s consisted of L-glutamic acid and N⁵-ethoxysilyl-L-glutamine residues which were prepared by the reactions of copoly (L-glutamic acid) containing succinimide ester with γ-aminopropyl-triethoxysilane. Their blood compatibilities were evaluated in terms of the adhesion behavior of blood platelet on surface was investigated by scanning electron microscopy. On the other hand, critical surface tensions(γc) and interfacial free energy between polymer surface and water (γsw) were also discussed. It was shown that the values of γc were 25-43 dyn/cm and γsw were 9-21 erg/cm² for all polymer samples. Results from these measurements, the γc were not the primary factors for studies of blood compatibilities, and blood compatibilities were improved with decrement of γsw.