Abstract: The effects of the amount of the ferrocene and ammonium chloride on the diameter distribution of hollow carbon spheres (HCSs) prepared in high-pressure argon were investigated. The morphology and diameter distribution of the HCSs were characterized by field-emission scanning electron microscopy which showed that the diameter of most HCSs ranges from 1 to 10 μm. The median diameter increases with the total amount of the reactants with the same weight ratio of ferrocene to ammonium chloride. When excess ammonium chloride is used, HCSs with a bimodal diameter distribution are prepared; however, the median diameter is almost the same. It is proposed that the aggregation of iron particles and liquid Fe(NH₃)₂Cl₂ templates, and the transformation of the iron amine complexes, in the process mainly influence the diameter distribution of the HCSs.