Carbon-coated Fe₃O₄ nanoparticles with tunable size were fabricated through a facile one-pot solvothermal method and employed as catalysts for Fischer–Tropsch (FT) synthesis to optimize the product distribution of hydrocarbons. Each spherical nanoparticle (Fe₃O₄@C) was observed to consist of a core–shell structure with an Fe₃O₄ core and a carbon shell. Furthermore, the Fe₃O₄ particle size can be effectively tailored by altering the initial dosage of glucose during catalyst preparation. Fe₃O₄@C with a suitable structure exhibited notable catalytic activity (CO conv. over 98%), high C₅–C₁₁ selectivity (46.6%) and superior stability in the FT synthesis. It was proposed that Fe₃O₄@C with a small particle size facilitated the formation of more active sites, which are responsible for the enhanced catalytic activity. However, the increase in particle and pore size promoted the selectivity towards C₅₊ hydrocarbons and lower olefins. This may provide a novel route to directly synthesize gasoline fuels during FTS.