Octenylsuccinic anhydride (OSA)-modified starches were prepared in granular form from waxy maize starch at two levels (3% and 9%, based on the weight of starch) of OSA reagent. The distribution of OS groups at five positions within a single starch granule was determined by Raman microspectroscopy. OSA-modified starch had one additional peak at 1670 cm-1, indicative of the carbonyl group in OS group bound to starch molecules. For each Raman spectrum obtained at a given position on a starch granule, the ratio of the band areas of carbonyl (1679 cm-1) to starch (2910 cm-1) was calculated. The highest level of OS groups was found on the granular surface. However, the level of OS groups in the center of granules also was high and nearly equal to that on the surface. The regions between the surface and the hilum of the waxy maize starch granules had reduced OS substitution.