The microstructure of electrodeposited nano-crystalline Au-Ni film was investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM). Carbon was found to be included in the deposit, and the state of bonding and dispersion of the carbon and its effect on the microstructure of Au-Ni film were studied. Nano-crystalline films were found to be composed of Au-rich crystalline phase and Ni-rich phase, and the latter consisted of crystalline and amorphous phases. The amorphous phase contained a large number of carbon particles measuring 5-20 nm. TEM and STEM examinations showed that the large carbon particles possess a graphite-like structure with some of the particles being arranged in arrays. Mechanism of the carbon inclusion was discussed based on the results of XRD and the composition analysis of Au-free Ni film electrodeposited from a cyanide-containing bath. These results showed that carbon is co-deposited with Ni as nickel carbide and also as carbon particles. It is concluded that the inclusion of carbon and the coexistence of Au and Ni are the conditions necessary for the formation of amorphous structure of the electrodeposited Au-Ni alloy film. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3579417] All rights reserved.