Abstract:The altitudinal gradient patterns of richness and species density of seed plants and their relationship with topographic parameters on Tongbiguan Nature Reserve in Southwest China were studied. The database of seed plants including their distribution ranges was constructed by field investigation many times, and the DEM (Digital Elevation Model) was derived from a topographical map of this region. The results showed that the species, genus and family richness increased rapidly at first and then decreased with the elevation increment, and those reached up to the maximum at middle altitude regions. The maximums of species and family richness appeared at the altitudes between 1400 m and 1800 m, and that of genus richness at the altitudes between 1000 m and 1400 m. When the area factors were eliminated, the taxa density decreased and increased firstly, then decreased and increased finally with the increment of elevation, up to peaks at the altitudes between 3000 m and 3400 m. The distribution patterns of species richness and species density were affected by the elevation gradient significantly which integrated many factors, such as moisture and energy conditions and so on. The altitudinal patterns of species richness at family, genus and species levels were consistent with the mid-domain effect hypothesis. It was took a question of "Logarithmic Model" in the calculation of the species density which was commonly used in the research of biodiversity at present.