Abstract: Compositional characteristics of pentacyclic tritepanoids in modern marsh sediments of southern Gansu is discussed by means of GC -MS. Five novel compounds of de -A-lupenes in 1 - 4 samples were found. By means of mass spectra, they are determined as: de - A - lup -18(19) - ene(two isomers), de -A -lup -12(13) -ene, de -A -lup -5 (10), 18(19) -diene and de -A-lup -5(10), 11(12)-diene. It is assumed that 3 - ketolupane and 3 -ke-tolupane acid are the precursor of the lupenes. De - A - lupenes are formed by ways of complete loss of ring A, resulting from a 3,4 cleavage and the last of carboxyl,de - A - lupanes acid are degraded products of lu-panoids during early diagenisis stage and also biomarkers indicating higher plant input. Hopanoids are composed of C₂₇,C₃₀,C₃₁,hopenes and C₂₇,C₂₉-C₃₂ hopanes. There is a higher propotion of Cu hopane in the 1 - 4 samples. C₃₁ and CM hopanes are only present at small concentration. Therefore it is prorposed that C₂₉ hopanes may mainly originated form higher plants, while high carbon number hopanoids from bacteria and aquatic organism. Isomerides of hopanes are composed of ββ-βα and αβ hopanes. ββ hopanes are predominant ; hopenes have reladtively higher concentration. These points reflect the characteristics of organic material during diagenesis stage. The discovery of C₂₉,C₃₀ ring D momoaromatic 8,14-secohopanes in the 1 - 4 samples is significant,which is presence may be related to acidic medium (pH=5) and weak oxidation-weak reduction enviromnent as well as activity of microbes in the modern marsh sediments of southern Gansu.