Rapid pyrolysis (similar to 10(4) K/s) experiments investigating lignite, bituminous coal and anthracite were conducted to study the effect of the thermal process on the coal structure. The interest was focused on the chemical structure variation occurring at the early stage of pyrolysis (0-500 ms) and high temperature (1173-1773 K), which includes the mass loss ratio, volatile content, element content and organic groups. A series of parameters was used to characterize the chemical structure, CH2/CH3 ratio, fraction of aromatic hydrogen and aromaticity (f(a)). The char's molecular size was determined based on both ring condensation degree (R/C)(u) and equivalent diameter of the aromatic monomer (D-C). This result demonstrated that these five chemical structural parameters (CSPs) increase with the pyrolysis time and temperature. The comparison of three types of coals reveals that the CSPs of lignite are lower and increase faster than those of bituminous coal and anthracite. (C) 2016 Elsevier Ltd. All rights reserved.