In order to increase the conductivity and sensitivity of boron-doped diamond (BDD) electrodes for oxygen sensors application, modification with gold nanoparticles (AuNPs) was performed. This study aims to synthesize AuNPs using allyl mercaptan as the capping agent and employ the AuNPs to modify BDD electrodes. Characterization of the synthesized AuNPs by using UV-Vis spectrophotometer showed the specific gold nanoparticles wavelength at about 510 to 580 nm, while the characterization by using Particle Size Analyzer and Transmission Electron Microscopy (TEM) showed that AuNP formed particles with an average size of 11 ± 4 nm. Modification of BDD surface with the synthesized AuNPs by using immersion technique under UV irradiation resulted in homogeneously distributed of AuNPs at the BDD surface with Au:C ratio around 2:98. Preliminary study of the AuNPs-modified BDD to determine dissolved oxygen solutions by using cyclic voltammetry technique suggested that the sensor has comparable capability to gold bulk electrode.