This study mainly focused on the removal of iron and manganese from water sources by a crossflow ultrafiltration (UF) process in combination with a prechlorination step using different chlorine dosage, along with evaluation of removal efficiencies for natural organic matter (NOM) and turbidity. For feed water containing 1.0 mg/L as Fe and 0.5 mg/L as Mn, substantial removal of Fe was achieved without chlorination, possibly because of the oxidation of ferrous iron to ferric iron by dissolved oxygen (DO) and consequent precipitation. However, there was no Mn removal in the absence of chlorine but with addition of chlorine, Mn removal increased dramatically and reached more than 80 % at a chlorine dose of 3 mg/L. In contrast, with a higher chlorine dose (e.g., 5 mg/L as Cl₂), further Fe and Mn removal was not significant, but serious membrane fouling was occurring. However, it was demonstrated that NOM removal substantially increased in the presence of Fe and Mn by chlorination. Presumably, it was believed that Fe or Mn oxides formed by chlorination played a role in NOM removal by adsorption.