Southwestern Africa contains diverse biomes. The amount of the annual precipitation of this area has been traditionally thought to be the most important controlling factor to the differentiation of the biomes. However, this territory experiences the summer rain type and winter rain type. Those two different precipitation patterns should result in the different mechanisms to control the distribution of the biomes. This study intends to clarify the relationships between the distribution of the biomes and the summer rain type and winter rain type, knowing the vegetation structure in terms of the growth forms of the dominant plants. Studies were conducted in the area southwestern Africa, including major biomes of the southern Africa. Grassland, Nama-karoo and Savanna appear in the area with the summer rain type. Both of Succulent karoo and Fynbos appear in the area with the winter precipitation type. The amount of winter precipitation of those two biomes is much higher than that of the other tree biomes. This higher amount of winter precipitation encourages the dominance of evergreen woods and succulents in those two biomes. Especially the succulents efficiently utilize the winter precipitation. This leads to the dominance of the succulents on those two biomes. Keywords: Deciduous shrub, Grassland, Savanna, Succulents, Summer rain, Winter rain.