Black carbon is a ubiquitous contaminant that is widely present in water sources, where, as dissolved (<0.2-µm) black carbon (DBC), it contributes to the pool of disinfection byproduct precursors. DBC, in the form of nano-scale particles, provides a large surface area to adsorb natural organic matter (NOM) and while NOM with large molecular weight (MW) is readily removed by coagulation, it is unclear whether the presence of different MW fractions of NOM adsorbed on DBC improves or impairs its removal. In this study, experiments have been conducted to investigate the interactions between DBC with different NOM MW fractions, and alum coagulant. The results found that as the MW of NOM decreased, the total organic carbon of adsorbed NOM on the DBC nanoparticles (NPs) decreased, whereas the DBC removal increased due to the increased destabilization of DBC NPs. As the MW of NOM decreased, the size of DBC with adsorbed NOM decreased, whereas the size of its flocs increased. As the MW of NOM decreased, the variation of carbon to oxygen ratio decreased, indicating the aggregation degree of DBC adsorbed with NOM increased. It was observed that as the MW of adsorbed NOM increased, there was a decrease in aggregation of the DBC, as well as the amount of aggregated DBC NPs detected in the flocs. The findings of this study provide important new information concerning the mutual interaction between NOM and DBC NPs and the effect of NOM on the fate of DBC during underground water treatment.