Microcalorimetry was used to investigate the microbial activity in three types of soil （orchard soil, crop soil, forest soil） in Wuhan, China, and to evaluate the influence of different concentrations of lead （Pb^2＋） on soil microbial activity. The experimental results revealed that due to different physical and chemical characteristics of the soils, soil microbial activity in three soil samples were in a descending sequence： orchards soil, crops soil, forest soil. Six levels ofPb viz. 0, 10, 20, 40, 80, 160 μg·g^-1 were applied in these soils, and the results showed that an in- crease of the amount of Pb^2＋ is associated with a decrease in microbial activity in the soils due to the toxic effect of Pb^2＋. In order to gain further insight of the sequential change of microorganisms, determination of colony forming units （CFU） was performed to provide a negative linear correlation between the heat effect and the respective number of microorganisms in the system.
