Small-angle neutron scattering was studied in Invar Fe-Ni-C alloys in comparison with the Fe-Ni alloy. Mossbauer measurement was carried out to indicate magnetic structure. The nonlinear SANS curves were analysed using a power law function and the Porod approximation and interpreted in terms of aggregates characterized by smooth surfaces or fractal properties. The size and shape of the inhomogeneities were estimated by using the Indirect Fourier Transformation analysis.