Laboratory simulation (pyrolysis) is an important approach for the study of petroleum formation and evolution. It is useful to determine the maturity levels under experimental temperature and time conditions. Mean random vitrinite reflectance (%Ro) is a parameter widely used to demonstrate source rock maturity. Confined pyrolysis experiments were conducted for simulating vitrinite maturation on six coal samples with initial hydrogen indices (HI) ranging from 155 to 259 mg HC/g TOC at 2 and 20 degrees C/h heating rates. Coal JC24 having higher initial HI (259 mg HC/g TOC) has relatively lower measured vitrinite reflectance (%Ro) compared with that of coals JC23 and JC25 having lower initial HI (i.e., 171 and 155 mg HC/g TOC, respectively), at the same thermal stress levels. For all six coals, the measured %Ro is lower compared to the predicted EASY%Ro at >1.5 EASY%Ro. The differences between the measured %Ro and calculated EASY%Ro are substantially greater at 20 degrees C/h compared to 2 degrees C/h at the same thermal stress levels. These results demonstrate that the measured %Ro values are influenced by the hydrogen index and heating rate, in addition to temperature and heating time, and suggest that EASY%Ro values may be more representative of thermal stress compared with the measured %Ro for pyrolysis experiments in a wide range of EASY%Ro (0.50-4.40).