New electroactive gold nanoparticles containing tetrathiafulvalene (TTF-Au nanoparticles) of 2-4 nm diameter have been prepared by the reaction of octanethiol-derivatized gold nanoparticles with a tetrathiafulvalenyl monothiol. Immersion of a gold electrode in a solution of the colloidal TTF-Au nanoparticles deposited the colloid films of them on the electrode surface which were remarkably stable under repeated electrochemical cycling. Thus, the TTF-Au nanoparticle films were firmly immobilized on the electrode by a simple self-assembly method without use of a terminal ligand like bifunctional aminosiloxane or mercaptosiloxane. A three-dimensional network of TTF-Au nanoparticles cross-linked by tetrathiafulvalenyl tetrathiol was assembled on a gold electrode.