Boron-doped diamond (BDD) was modified with copper and gold particles by using an electrodeposition technique to improve its catalytic effect on CO2 reduction in a flow system. The system was optimized based on the production of formic acid by the electroreduction process. At the optimum applied potential of −1.0 V (vs. Ag/AgCl) and flow rate of 50 mL min−1, the copper-gold-modified BDD produced formic acid at the highest rate of 4.88 mol m−2 s−1 and a concentration of 15.93 ppm, while acetic acid was produced with a rate of 0.11 mol m−2 s−1 and a concentration of 0.47 ppm. An advantage of the flow system using the modified BDD was that it was found to accelerate the production rate of acetic acid as well as to decrease the reduction potential of CO2. Furthermore, better stability of the metal particles was observed when using mixed copper-gold modification on the BDD surface than single modification by either metal. The results indicated that a flow system is suitable to be employed for electroreduction of CO2 using the bimetal-modified BDD electrodes, especially with copper and gold as the modifying particles.