Scaling of reverse osmosis (RO) membrane surface is one of the main problems in desalination processes. To mitigate scales, organic anti-scalants are often used. If the dosages of anti-scalants are reduced, by using other much cheaper scale inhibitors, RO running cost will decrease greatly. The present paper investigated the inhibition of CaCO3 precipitation by zinc ions in RO system. The results show that the zinc ion concentration of 2mg.L-1 was able to exert a marked suppression effect on both bulk precipitation of CaCO3 and on membrane scaling on waters of moderate hardness.