Siam Queen Basil and Thai Basil differ in their appearance. Thai basil leaves are smaller and more narrow than the larger, circular leaves of sweet basil. On the other hand, Siam Queen Basil has green foliage with attractive lavender flowers that make it a popular ornamental plant in addition to its culinary uses. The leaves of Siam Queen Basil are longer and wider than those of Thai Basil, with leaves that are 4 inches long and 2 inches wide.
 
Siam Queen Basil and Thai Basil also differ in their taste. Thai basil has a spicy flavor with an anise or licorice-like taste. On the other hand, Thai Basil has a more mild, peppery, and sweet flavor. Thai Basil is commonly used as the main ingredient in Genovese-style pesto.