Reverse Osmosis is a possible process production of drinking water from saline or freshwater, in which water is forced through a semi permeable membrane resulting in production of pure water with very low ion concentrations. This water is then remineralized to prevent corrosion and make it suitable for consumer usage. It is reported that at higher temperatures, for instance in boilers, scaling occurs in an earlier stage in drinking water produced by RO than water produced with conventional water treatment plant. Presumed is that the absence of certain materials removed by RO during filtration is the cause of this early scaling. One of these substances is humic acids. In this research the effect of addition of several concentration of HA on scaling is measure at high temperature and calcium concentration of approximately 20mg/l and 40mg/l. The results show that dosing of at least 3 mg/L of humic acids can significantly reduce scaling in RO permeate water. In water with lower Ca2+ concentrations no scaling was measured thus the experiment was repeated with a higher initial Ca2+ concentration.