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Vegetation composition, main plant communities and altitudinal vegetation zones at Asir Mountain southwest Saudi Arabia were studied, and in relation to various environmental factors including climatic, geological, ecological and topographic. A total of 74 plots were sampled according to Braun-Blanquet method along an altitudinal gradient. Floristic and environmental (topographic, geological and climatic) data were collected and analyzed using numerical classification and ordination. Two main climatic regions, four altitudinal vegetation zones and fifteen vegetation groups were identified: (1) two groups in an arid coastal plains mainly on an alluvial substratum between 0 and 200m (2) seven groups in the interspersed hills and plateaus of Tihamah arid region from 200 to 1100m; (3) three groups in the escarpments from 1100 to 2000m in the semi-arid region; and (4) two groups in a semi-arid high mountains zone from 2000m to the summit. Geo-altitudinal gradient was affected by the climatic factors (rainfall and temperature) and vegetation structure (species richness and coverage) on a regional scale, while edaphic factors played the leading role in the distribution of different vegetation groups on a local scale. These findings coincide with those reported about the distribution of vegetation at other tropical mountains. Human activates have a significant effect on modifying the distribution and abundance of plant species.