The widespread presence of Fe particles and natural organic matter in drinking water distribution systems can significantly affect tap water quality, contributing to aesthetic issues and potentially generating harmful disinfection byproducts (DBPs). This study reveals that, when combined with humic acid (HA), Fe particles substantially increased DBPs formation during chlorination. Fe particles (particularly preformed Fe particles) significantly increased haloacetic acid (HAA) formation by activating the persistent free radicals (PFRs) in the HA. At 5 mg/L HA, the levels were 2- to 2.5-fold, respectively, those in the HA-only control; and at 10 mg/L HA, they were 1.3- and 2.2-fold, respectively, those in the HA-only control. Iron particle surface accumulated PFRs could generate hydroxyl radicals, facilitating the decomposition of HA into smaller molecules, which were more reactive with chlorine disinfectants, thus elevated the DBP formation including both known and unknown N-DBPs and Cl-DBPs. The DBP promotion effect of in-situ formed iron particles was much less than that of preformed iron particles although both in-situ formed and preformed iron particles could accumulate PFRs from HA. In-situ formed particles primarily accumulated carbon-centered PFRs, while preformed particles accumulated oxygen-centered PFRs. To mitigate these risks, it is crucial to control Fe corrosion in drinking water distribution systems. Ensuring safe drinking water requires the optimization of pretreatment processes such as coagulation and filtration to remove HA and Fe particles. Furthermore, alternative disinfection methods that minimize DBP formation are required.