The present paper deals with the composition of the newly formed humus derived from various plant materials decomposed in a Quaternary red clay and in a in a Xiashu loess for 3 years.Experiment results showed that 1.the ratio cf C/org.N and fractional humus composition of newly formed humus varied with the kinds of plant material from which it was derived:2.as compared with those formed under submerged conditions,the newly formed humus formed under upland condition had higher C/org.N and hexoses/pentoses ratios and sugar content as well as a higher extractability of humic substances;3.the newly formed humus formed in Xiashu loess had a lower C/org.N ratio,but higher HA/FA and hexoses/pentoses ratios than that in Quaternary red clay.The role of clay mineral in the formation of humic substances is discussed.