A metallic Co nanoparticle catalyst embedded in a carbon matrix was synthesized through a facile solvothermal method and subsequent thermal treatment. The synthesized Co–C precursors were calcined in N₂ and limited air atmosphere respectively, which were applied for ammonia decomposition. It was found that different thermal treatments have a significant effect on the catalyst microstructure, particle size, components and catalytic performance. Compared to the Co@C-700-N sample obtained in N₂ flow, the CoO_x@C-700-A catalyst obtained in limited air atmosphere showed better activity with a ∼55% NH₃ conversion (500 °C and GHSV = 15 000 h⁻¹). Moreover, the structure sensitivity of NH₃ decomposition over Co catalysts is proposed over a series of CoO_x@C-A catalysts. With an increase of Co particle size, the catalytic activity showed an obvious decrease.