Pairs of calcareous and non-calcareous soils under similar vegetation were collected from Taian, Nanjing and Kunming districts and the fractional composition of humus and structural characteristics of humic acids of these soils were studied. Results showed that. the HA/FA ratio of calcareous soil was consistently higher and the relative content of mobile HA was significantly lower than those of non-calcareous soils in the same location. Moreover, the humic acid extracted.from the calcareous soil was hjgher in aromaticity and content of carboxyl group and lower in contents of total nitrogen and amide nitrogen as compared with that extraded from the non-ealcareous soil. It seems that the chemical environment of calcareous soil is favorable not only for the decomposition of organic matter, but also for the polycon-densation of phenolic compounds with nitrogen-containing compounds, as well as the deamination of humic acids.