Abstract: Drainage ditches play an important role in agricultural landscapes and in the maintenance of plant species diversity. The Sixth drain area (41.051-41.286N, 108.270-108.326E, mean altitude 1027 m) locate in the middle part of Hetao Irrigation Region; the net drain area covers 26 km2. There are seven levels of drainage ditches in the Sixth drain area (i.e. the main drainage ditch, feeder drainage ditch and sub-feeder drainage ditch, etc.). Main drainage ditches have longer water retention times than feeder and other drainage ditches during the irrigation period from May to No-vember. Special heterogeneity is also one of key factors impact on species diversity. All of these may have important effects on plant species composition and diversity as well as spatial distribution. However, there is a lack of information about how plant species exist in this area as well as the characteristics of different kinds and sizes of drainage ditches. A comprehensive investigation of drainage ditches in the Sixth drain area was conducted to answer two questions: 1) how many plant species exist in this area; 2) does plant species -diversity differ among different levels and positions of drainage ditches. The field work was conducted in August and September of 2010. A total of one main drainage ditch, three feeder drainage ditches and 9 sub-feeder drainage ditches were selected for the survey. Five transects were lain equidistantly from upstream to downstream along main drainage ditch, and three transects were lain at upstream, mid-stream, and downstream locations along each feeder and sub-feeder drainage ditch. The mean depth of water, width of the water surface, and slope length in each ditch were recorded. At each transect, 2-3 plots were set perpendicularly from the bottom of the ditch to the bank to measure herbaceous species, and a plot was set up on the slope to measure larger herbaceous and shrub species. The size of sampling plots varied from 0.25 m2 (0.5 m0.5 m) for herbaceous spe-cies to 25 m2 (5 m5 m) for large herbaceous and shrub species. In total, 41 transect and 123 sample plots (101 for herbage and 22 for shrub sample) were placed in the Sixth drain area. In each plot, species number and composition were recorded. Unknown plants were photographed and collected, and taken back to the laboratory for further identifi-cation. The -diversity of plant species, which was measured by the Shannon?Wiener Index, Richness Index, Pielou Evenness Index and Simpson index, were compared between drainage ditches of different sizes and positions. The re-sults showed that 43 species representing 41 genera and 22 families were founded by the present investigation. Gramineae, Compositae and Leguminosae were the largest families in the flora. The main vegetation type was herbage and was dominated by Phragmites communis Trin. The Shannon's Index, Simpson Index, Richness Index, Pelion Index of main drain, feeder drain and sub-feeder drain were respectively 0.79, 0.37, 3.20 and 0.55; 0.90, 0.54, 4.37 and 0.62; 0.89, 0.53, 4.25 and 0.63. Plant species in the Sixth drain area was primarily composed of single species and single genera plants. The main drainage ditches supported lower species diversity overall and fewer unique species than either the feeder or sub-feeder ditches at both the local and regional level. Species diversity tended to be the highest on the slope of the ditch, and lowest in bottom of the ditch. Special heterogeneity is likely the main factors affecting plant species diversity in this area.