Cytochrome c (cyt.c) was cross-linked with glutaraldehyde (GA) intramolecularly to stabilize the conformation at the electrode surface and to improve its electron transfer process in PEO oligomers. Mass spectra of these cyt.c proved that cross-linking with GA was formed intramolecularly. The cyt.c cross-linked with GA (GA-cyt.c) showed larger peak current and charge than those for PEO-modified cyt.c in PEO oligomers. Redox response of GA-cyt.c in PEO oligomers was improved by increasing the average degree of cross-linking. Although GA-cyt.c was adsorbed on the ITO electrode surface, it showed clear redox response in PEO oligomers. Intramolecular cross-linking was concluded to be effective to prevent large conformational change of cyt.c which was induced by the adsorption at the electrode surface, and to provide better redox response in the PEO oligomers.