A new generation of high performance martensitic steel hardened by FeNiAl intermetallic nanoprecipitates has been investigated using conventional and advanced Transmission Electron Microscopy (TEM). Cs-corrected high resolution TEM has been used to provide a fine and accurate microstructural characterization: the shape, size and crystallographic characteristics of the nanoprecipitates have been identified. TEM in situ straining tests have been performed to determine the relevant parameters, which control the deformation. The nanoprecipitates are observed to create anchor lines acting against the propagation of the mobile dislocations and to be crossed by shearing process. In addition, in situ TEM experiments have allowed to measure directly the average distance between these nanoprecipitates along the dislocation lines. This value is of great interest as it is the more accurate data to be used in the calculation of the stress associated with the structural hardening. The strength of the alloy due to these nanoprecipitates has been also determined. It is in good agreement with the value deduced from macroscopic mechanical tests.