Hulun Lake, the fifth largest lake in China, is located in the west side of Hulun Buir Prairie.It was seldom studied and understood before due to the remote border district.The observation and sampling of the modern sediment in Hulun Lake has been carried out and the mineral composition and the sedimentation rate analysed recently.The surficial sediments of Hulun Lake are distributed unevenly, for the lake is linked to the influxes of the Herlen River, the Orxon River and the ancient Harlar River.Grainsize analysis of 7 sediment samples in the northwest demonstrates that the median size of most sample is 7.0-9.2φ, falling in the size fraction of clay-very fine silts.Mineral identification by microscope and XRD analysis for the sediment shows that the sediments are such allogenic minerals as quartz, feldspar, mica and heavy minerals and authigenic minerals, mainly calcite.Carbonate content of the sediment samples ranges from 10.1 % to 17.6% indicated by chemical analysis, which is linked to higher salinity and supersaturation to carbonate of the lake water.^2l0Pb dating was tested for a short core in the central pwirt (site HL8) of the lake and the sedimentation rate was calculated using CIC model.The average sedimentation rate is 0.25 cm/a above lOcm-depth of the core and 0.05 cm/a in 10-12cm, implying higher sedimentation rate since 1959 A.D.Based on the hydrologyical data, the lake level rose gradually begiiiing from the 1950s and up to its historical highest in the 1960s.It is inferred that the influx sediment amount increased as the lake level rose.