Abstract: The steranes and terpanes in the source rocks from the Lower Silurian Longmaxi Formation in northern Guizhou are determined with the aid of gas chromatography and chromatography-mass spectrographic analysis.The analytical results suggest that the n-alkanes in the source rocks display a back single-peak pattern,with the main peak carbon values ranging from C₂₂ to C₃₀.The Pr/Ph ratios vary from 0.48 to 0.64;Pr/nC₁₇ ratios vary from 1.35 to 4.22,and Ph/nC₁₈ ratios vary from 0.92 to 0.97.The pregnane and homopregnane are very rich in steranes.C₂₇,C₂₈ and C₂₉ display a regular V-type distribution pattern(C₂₇ > C₂₈ > C₂₉),with a C₂₇ dominance.The sterane/aromadendrane ratios range between 0.57 and 0.71.The terpanes are characterized by high tricycloterpane values,low aromadendrane values and high gammacerane indexes.All the values for the maturity such as OPE<1,Ts/(Ts + Tm) ratios ranging between 0.42 and 0.48,C3122S/(S + R) ratios ranging between 0.6 and 0.66,and C₂₉ ββ/(ββ +αα) ratios ranging between 0.56 and 0.58 indicate the stratified water bodies and highly reduced conditions during the Longmaxian.The organic matter dominantly came from planktons and algae,and has been subjected to a certain degree of biodegradation.The thermal evolution of the source rocks has reached up to the stages of high maturation and overmaturation.