Abstract: Nanoscale zero-valent iron (nZVI) is a newly developed method for in-situ remediation of groundwater in recent years. To overcome easy oxidation and aggregation, modification on nZVI is widely adopted. In addition, the ubiquitous natural organic matters (NOM) in water also affect the scatter and reaction activity of nZVI. Therefore, it is of significance to study the aggregation effect of different kinds of nZVI in water by in-situ measurement method. Besides commercial nZVI (RNIP), normal nZVI, carboxymethyl cellulose coated nZVI (C-nZVI) and bentonite supported nZVI (B-nZVI) was synthesized. Nano particle size/Zeta potential analyzer based on the Dynamic Light Scattering (DLS) technique, Transmission Electron Microscope (TEM) and UV-Vis spectrum of sedimentation were employed to characterize the aggregation behavior of above four types of nZVI in the presence or absence of humic acid (HA), a representative NOM. The results showed that the dispersion stability of C-nZVI and B-nZVI was improved significantly after modification and the aggregation precipation was suppressed. And size distribution of aggregates stabilizes at below 1000 nm. Humic acid adsorbed onto the surface of nanoparticles, which can increase the electrostatic force to prevent aggregation of nZVI. The effect of HA on B-nZVI is most obvious for B-nZVI aggregates with diameters below 100 nm and the sedimentation was very slow, indicating the dispersion performed the best among all kinds of nZVI. It can be concluded that the combination of DLS technique and TEM to characterize nanoparticles is a valuable approach for more comprehensive information.