In this study, the synergetic effects in a Fenton-like system catalyzed by nano zero-valent iron (nZVI Fenton-like system) were studied using nitrobenzene (NB) as a model contaminant. The results showed that homogeneous and heterogeneous Fenton processes existed simultaneously in the nZVI Fenton-like system, and a synergetic removal effect between these processes played a considerable role in NB removal. Through quantitative analysis, 36.5% of NB degradation was attributed to the synergetic degradation effect, which was caused by a synergetic catalytic effect between nZVI and dissolved iron ions (Fe3+ and Fe2+). In the bulk solution, the Fe3+/Fe2+ redox rate was accelerated by nZVI, resulting in the efficiency improvement of homogeneous catalysis; in the surface of nZVI, these dissolved iron ions promoted the electrons transfer from nZVI core to shell, enhancing the efficiency of heterogeneous catalysis. The synergetic catalytic effect also improved the utilization-rate of H2O2 by reducing the decomposition caused by Fe3+/Fe2+ redox compared to that in the homogeneous Fenton system. Based on these results, a possible mechanism of synergetic effects in the nZVI Fenton-like system was proposed. These results could provide insight into an nZVI Fenton-like system.