The influence of NH4+ on the preparation of carbonaceous spheres by a hydrothermal process was studied. A scanning electronic microscope and laser particle size analyzer were used to observe and measure the morphology and size distribution of the spheres. Fourier transform infrared spectroscopy was used to analyze the functional groups of the spheres. The results show that the addition of NH4+ has a significant influence on the morphology, size, and yield of the spheres. The slight addition of NH4+ greatly increases the diameter of the spheres and simultaneously improves their dispersion. With the change of NH4+ concentrations, the as-received spheres have sizes from 0.2 to 20 mu m in diameter. The Fourier transform infrared spectroscopy results indicate that the C = O group disappears in the samples with NH4+ addition, different from the obvious peak at 1700.2 and 1305.6 cm(-1) in the samples with no NH4+ addition. A possible mechanism of the formation and growth of the spheres with the addition of NH4+ is also proposed in the paper.