Charred plant fragments are frequently observed in soil. However, their structure has not been elucidated. In the present study, charred plant fragments detached from soil by HF treatment and collected by heavy liquid separation were characterized by X-ray diffraction (XRD) analysis. Three Andosols from the northeastern, central and western districts of Japan were used. Supportive information was obtained by solid-state 13C nuclear magnetic resonance (NMR) and δ13C analyses. In the XRD analysis, the size of the carbon (C)-layer planes in the charred plant fragments ranged from 0.96 to 1.92 nm, corresponding to 14–52 ring condensed aromatic structures. The size distribution of the C-layer planes did not differ largely among the three soils. A minor effect of vegetation on the composition of the condensed aromatic structures in the plant charred fragments was deduced from differences in the content of the 1.92 nm C-layer plane and δ13C. The relative content of condensed aromatic structures tended to be larger in the sample with more aromatic C content, which suggested that decomposition of aliphatic moieties is a cause of enrichment of condensed aromatic structure.