The two coal components, barkinite and vitrinite, were selected from the same coal, Mingshan coal, in southern China. The chemical structural transformations of these two components upon heat treatment in situ from 200 to 500 degrees C were examined by a high-resolution transmission elettron microscopy (HRTEM) technique. The physical appearance changes were observed by polarized light microscopy with a heated stage. At 200 degrees C, barkinite and vitrinite have no clear change in physical appearance. Most of the layers are poorly orientated in chemical structure. A rounded-off edge of the barkinite particle was observed at 250 degrees C, and that of vitrinite was observed at 300 degrees C. The particle sizes were drastically reduced from 450 to 500 degrees C. A massive amount of oily material was exuded from the barkinite particle. Meanwhile, the orientation in the aromatic layer of barkinite and vitrinite was clearly improved. The changes of the aromatic size of barkinite and vitrinite upon heat treatment were obvious. The naphthalene abundances decreased from 200 to 350 degrees C and increased at the range of 400-500 degrees C. The abundances of naphthalene reached a maximum value at 450 degrees C. The obvious increase of the 3 x 3 fringe and 4 X 4 fringe at 200-350 degrees C and decrease at 400-500 degrees C were shown. The abundances of the 2 x 2 fringe were different. Furthermore, differences existed on the structural changes of barkinite and vitrinite when heating. First, barkinite becomes the obvious orientation of the layer at almost 375 degrees C, and vitrinite becomes the obvious orientation of the layer at 400 degrees C Second, barkinite has a bigger naphthalene abundance and lower 2 X 2 fringe and 3 X 3 fringe at 450 degrees C than vitrinite. The differences in chemical structure of barkinite and vitrinite played a significant role in investigations for the different thermal behaviors of these two components and bark coal.