At first, C-dots/manganese oxide nanocomposite was synthesized chemically. The presence of MnO nanostructures and C-dots in the synthesized nanocomposite was confirmed by using of XRD, EDS, FTIR and TEM. The proposed method for synthesis is easy and cheap. The MnO/C-dots nanocomposite shows oxidase-like activity, which is higher than oxidase-like activity of MnO nanoparticles. In this manner, detection of ferrous ion was possible by using of a competitive reaction between 3,3′,5,5′-tetramethylbenzidine and Fe ion in the presence of the synthesized nanocomposite. So, a sensitive and selective colorimetric sensor for determination of Fe ion was introduced. The linear range and detection limit for determination of Fe ion were obtained as 0.03–0.83 μM and 0.03 μM, respectively.