Quartz crystal microbalance (QCM) technique has been applied to the direct mass measurement of a self-assembled monolayer of octadecylmercaptan (OM) on a gold surface deposited onto a quartz-crystal. Repeated rinsing of the modified electrodes with used solvent was conducted to get self-assembled monolayers of OM. From the resonance frequency change, 2.5 nm molecular occupied area of OM on gold was obtained, The advantage of the combination of QCM and electrochemical methods was emphasized for the characterization of self-assembled monolayers.