The Fisher-Tropsch (FT) synthesis process presents a way to produce extremely pure liquid fuels from natural gas and other sources. Supported cobalt nano particles are widely used as FT catalyst systems. To optimize the efficiency of these catalysts and the FT process, a fundamental understanding of the catalysts structure and catalytic activity must be obtained on the nano scale. Transmission electron microscopy (TEM) is a powerful technique for these studies, but due to reactive nature of cobalt nano particles in-situ experiments are required. Here we have investigated the in-situ reduction of cobalt nano particles on alumina support. This experiment has performed with a state-of-tha-art environmental cell electron microscope in electron nanoscopy centre in Denmark.