The influence of cathodic hydrogen charging on the nanohardness of austeno-ferritic stainless steel was investigated. As it is well known that hydrogen behaves differently in the ferrite (α) and austenite (γ) phases - in the fcc γ phase, hydrogen has a higher solubility but a lower diffusivity than it has in bcc α phase - the hardness changes of both α and γ grains before and after hydrogen charging were investigated. Because of the fact that only thin film on the steel surface suffered from hydrogen embrittlement, it was necessary to use the fine nanoindentation technique. Therefore the nanoindentation measurements using the Atomic Force Microscope were performed. Three various states of samples were examined: initial state, just after hydrogen absorption and after hydrogen desorption. It was shown that there are distinct changes of nanohardness between every state and every phase. Generally, the nanohardness of the phase is higher than in case of the phase. The increase of nanohardness of both phases after hydrogen absorption and its decrease after hydrogen desorption was noticed.