In this study, carbon dioxide (CO2) was used as an inhibitor of scale production on the surface of RO membrane. In order to compare the effects of CO2 injection on scale production, four RO modules: 1) without CO2 injection and anti-scalant (RO module #1), 2) with only CO2 injection (RO module #2), 3) with only anti-scalant (RO module #3), 4) with both CO2 injection and anti-scalant (RO module #4), were operated for 60 days under constant flux mode. The trans-membrane pressure (TMP) was observed to decrease significantly in RO modules with CO2 injection as compared with the other RO modules. When the feed water pH was controlled at 5.0 by injecting CO2, the maximum TMP in RO modules #2 and #4 was founded to decrease by 42 aㅋnd 40%, respectively. Moreover, the Ca2+ concentration in the concentrate was 20mg/L lower in RO modules without CO2 injection which is attributed to the scale formation on the surface of the RO membranes. The SEM-EDS analysis further showed a serious fouled RO membrane surface in RO modules #1 and #3.