Site-selected and size-controlled iron nanoparticles were prepared on coplanar surfaces via microcontact printing of SAM-modified Au/mica electrodes and controlled-potential electrolytic reactions using ferritin biomolecules. Ferritin molecules packed like a full monolayer on 6-amino-1-hexanethiol (AHT)- and 11-amino-1-undecanethiol (AUT)-modified Au/mica surface via electrostatic interactions, which did not depend on the chain length of the amino terminal alkane thiols. After heat-treatment at 400 degrees C for 60 min, iron oxide nanoparticles (ca. 5 nm in diameter) derived from ferritin cores were observed at the Au/mica surface by atomic force microscopy (AFM). On the study on the electrochemistry of ferritin immobilized onto AHT- and AUT-modified Au/mica electrodes, the redox response of the ferritin immobilized AHT-modified electrode was clearly observed. On the other hand, no redox peak for ferritin was obtained at the AUT-modified electrode. The electron transfer between ferritin and the electrode through the AUT membrane could not take place. The difference in the electrochemical response of ferritin immobilized onto AHT- and AUT-modified Au/mica was caused by the chain length of the amino terminal alkane thiols. Uniform patterns of AHT and AUT on the Au/mica electrode surface were performed by use of a poly(dimethylsiloxane) (PDMS) stamp. After the immobilization of ferritin onto both AHT- and AUT-modified electrode surfaces, the modified electrode was applied to a -0.5 V potential for 30 min in a phosphate buffer solution. After this procedure, the PDMS stamp patterning image appeared by scanning electron microscopy (SEM) image. The SEM results induced by the size change of the ferritin core consisting of iron(III) by electrolysis.