Starch is the most widespread and abundant storage carbohydrate in crops and its production is critical to both crop yield and quality. In regard to the starch content in the seeds of crop plants, there is a distinct difference between grasses (Poaceae) and dicots. However, few studies have described the evolutionary pattern of genes in the starch biosynthetic pathway in these two groups of plants. In this study, therefore, an attempt was made to compare evolutionary rate, gene duplication, and selective pattern of the key genes involved in this pathway between the two groups, using five grasses and five dicots as materials. The results showed 1) distinct differences in patterns of gene duplication and loss between grasses and dicots; duplication in grasses mainly occurred before the divergence of grasses, whereas duplication mostly occurred in individual species within the dicots; there is less gene loss in grasses than in dicots, 2) a considerably higher evolutionary rate in grasses than in dicots in most gene families analyzed, and 3) evidence of a different selective pattern between grasses and dicots; positive selection may have occurred asymmetrically in grasses in some gene families, for example, ADP-glucose pyrophosphorylase small subunit. Therefore, we deduced that gene duplication contributes to, and a higher evolutionary rate is associated with, the higher starch content in grasses. In addition, two novel aspects of the evolution of the starch biosynthetic pathway were observed.