In order to investigate the effect of total hardness and ionic strength on the coagulation of natural organic matter, different Al coagulants -conventional Al salt (AlCl3), polyaluminum chloride (PAC) and Nano polyaluminum chloride (Nano-PAC) - were used to treat humic acid (HA) in this study. The effects of total hardness and ionic strength on coagulation performance and coagulation kinetics of different aluminum were studied. Results show that when total hardness increases from 10 mg L-1 as CaCO3 to 160 mg L-1 as CaCO3, the highest HA removing efficiency of HA water decreases from 94.2% to 93.6%, while the highest HA removing efficiency in HA-kaoline water treatment increases from 94.3% to 95.3%. When ionic strength increases from 0.05 mol L-1 to 0.25 mol L-1, the highest HA removing efficiency of HA water decreases from 92.3% to 88.4%, while the highest HA removing efficiency in HA-kaoline water increases from 88.1% to 85.6%. Nano-PAC, which is thought of as a highly efficient aluminum salt in other studies, does not perform well for removing HA from water. (C) 2010 Elsevier B.V. All rights reserved.