Structural changes of Kuwait asphaltenes and resins by hydrodemetallization (HDM) reaction were investigated with laser desorption mass spectrometry, H-1 and C-13 NMR, and elemental analyses. Molecular weight (MW) for both asphaltenes and resins gradually decreased through HDM reaction. But their MW distributions (MWD) were different from each other. Namely, the MWD of resins became narrow while that of asphaltenes became polydispersed with an increase of HDM temperature. It was found that a steep increase of aromaticity around 400 degrees C was due to the shortening of alkyl side chains and the decrease of those numbers for resins and asphaltenes, respectively. For asphaltenes, the increase in internal quarternary aromatic carbons and the decrease in external ones were observed up to 410 degrees C, which indicates the shift of the aromatic skeleton in asphaltenes toward peri-type.