Acetylated potato and sweet potato starches were fractionated according to granule size. From the fractions obtained amylose and amylopectin were isolated and characterized with respect to degree of substitution (DS) and degradability with α-amylase, β-amylase and amyloglucosidase. The DS of the amylose populations of differently sized granule fractions was quite constant. In contrast, the DS of the amylopectin populations originating from the differently sized fractions increased with ecreasing granule size. The acetylation was confirmed to occur throughout the amorphous regions, and only take place in the outer lamellae of crystalline regions of starch granules. The model of granular starch acetylation was proposed. The amylose populations isolated from small size granule fractions of the acetylated starches tested were less susceptible to all the enzyme digestions than the amylose originating from the large granule fractions, even though the DS was similar. The acetyl populations groups over the amyiose molecules are more heterogeneously distributed and located more closely to the non-reducing ends for amylose originating from small size granule fractions when compared to amylose from lager sized granules.