Abstract: The study focused on plant diversity and species richness in each landform in the southwestern region of Saudi Arabia. One hundred and forty five species were recorded, belonging to 39 families. The most dominant families were Compositae (20 %), Graminae (14.5 %), Leguminosae and Chenopodiaceae (7 %). The contribution of the study area for ferns (25 %), gymnospermae (10 %), dicot (12 %) and monocot (11.9) of the total species in Saudi Arabia. The maximum number of plant species was attained in wadis followed by sandy plain habitats (111 and 109 respectively). The highest species richness/ habitat (111 sp./habitat) was recorded in wadis, while the lowest (56 sp./habitat) was recorded in rocky ridges . Beta diversity was estimated as the ratio between the total number of species and species richness (alpha diversity) where, there was little variation among four habitats (e.g. maximum ratio was 15.1 in sandy flat plateaus and minimum was 10.0 in both sandy rocky ridges and rocky ridges). About 10, 12, 12, and 25 % of the ferns, gymnospermae, dicotyledoneae, and monocotyledoneae were respectively the flora of Saudi Arabia. On the other hand, about 24 %, of the flowering plant occur in the southwestern region of Saudi Arabia.