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Electrical detection of sandwich immunoassay using indium tin oxide (ITO) interdigitated electrodes on a biochip is developed. Gold nanoparticles are utilised as the indication of the antibody/antigen/antibody complex on the electrodes and the gold enhancement process is for signal amplification. The electrical detection of the concentration of target antigen in sandwich immunoassay has been demonstrated. ITO interdigitated electrodes were fabricated on a glass substrate and worked as the detection sites. The capture antibody was first immobilised on the detection sites. The target antigens of certain concentration and detection antibody were applied, respectively. Then, sandwich-like antibody/antigen/antibody complex was constructed on the detection sites. Gold nanoparticles were utilised to bind the sandwich complex through biotin-streptavidin linkage. Then, gold enhancement process was performed for enlarging the size of the gold nanoparticles physically. Therefore a conductive layer was formed across the electrodes. The presence and the concentration of the target antigen can be determined by the resistance value across the electrode measured by an ohmmeter directly. The resistance values showed from 1200 to 700 Ω when the target antigen was in the concentration from 20 to 160 ng/ml. The proposed electrical sandwich immunoassay is simplicity, low cost and miniaturisation; therefore it is possible to be integrated into a microfluidic system and potentially be developed a portable diagnostic device.