Curvature is prevalently existed in the world of carbon materials (e.g., fullerenes, buckyl bowls, carbon nanotubes and on-ions), but traditional C2-addition mechanisms fail to elucidate the mechanism responsible for the formation of carbon curvature starting from a pentagonal carbon ring in currently available chemical-physical processes such as combustion. Here we show a complete series of nascent pentagon-incorporating C5-C18 that are on-line produced in the flame of acety-lene-cyclopentadiene-oxygen and in situ captured by C60 or trapped as polycyclic aromatic hydrocarbons for clarifying the growth of the curved subunit of C20H10. A mechanism regarding C1-substitution and C2-addition has been proposed for understanding the formation of curvature in carbon materials, as exemplified by the typical curved molecule containing a single pentagon completely surrounded with five hexagons. The present mechanism, supported by the intermediates char-acterized with X-ray crystallography as well as NMR, has been experimentally validated for rational synthesis of curved molecule in commercially useful combustion process.