Soil health is an important factor in sustainable agriculture. Organic matter supplement is one way to ensure the soil’s health. Contamination of soil by toxic elements is an important issue since it affects the health of organisms both directly and indirectly. Cadmium and lead pose the most risk to human. Adding organic materials to soils contaminated by heavy metals is considered to be an usual remediation practice. Humic substances are the major components of soil organic materials. Leonardite is a type of coal rich in humic substances. New sources of organic material can be useful for supplying soil humus, however, required to verify that these materials are biologically suitable for the soil. Moreover, these substances can be used to reduce or even control the contamination of toxic metals in soil. The objectives of this work is to study the effects of humic acid and Leonardite on some biological properties of soil (first experiment), and the inhibitory effect of heavy metals on the activity of some enzymes with and without the presence of these substances (second experiment). Two sources of humic substances used in this research were a leonardite sample obtained in Iran and a humic acid derived from the same leonardite, applied to a soil at three levels: 0 (Control), 10, and 20 grams of carbon per kilogram of soil. The treatments were applied on two different soils: Shervedan and Langroud. 200 grams of each treatment air dried after 2 months incubation in 25 ? C. In the first experiment, the responses of carbon and nitrogen mineralization, soluble organic nitrogen and also the activities of the urease, acid phosphatase and alkaline phosphatase enzymes to leonardite and humic acid were measured. In the second experiment, the inhibitory effects of lead and cadmium contamination on activities of the named enzymes were measured by inhibition percent (IP) method. The results of the first experiment showed that the presence of leonardite and humic acid increased the carbon and nitrogen mineralization and decrease the urease activity in the soils. The effects of humic acid and leonardite on the activity of acid and alkaline phosphatase were different in the soils. Nitrogen mineralization was positively correlated with carbon mineralization (r=0.891 *** ) and urease activity (r=0.714 *** ), but had no correlation with the other measured parameters. Soluble organic nitrogen was negatively correlated with acid phosphatase activity (r=-0.878 *** ). The activity of urease and alkaline phosphatase enzymes were positively correlated (r=0.540 *** ). Based on the second experiment results, using of humic acid and leonardite had no influence on the inhibitory effects of lead and cadmium on uease, acid phosphatase and alkalin phosphatase activity in Shervedan soil. Using of humic acid and leonardite in Langroud soil, reduced the inhibition effects of lead and cadmium on urease activity, but had no effect on the activity of acid and alkaline phosphatase enzymes. The results showed that the effects of humic substances on soil biological properties were different when the type and levels of the used materials and the soil type changed. Increasing the microbial population and activity was one of the outcomes of using these materials which can improve the carbon and nitrogen cycles in soil. Humic substances have inhibitory effects on urease activity but stimulate the enzyme activity in the presence of lead and cadmium. There were no results to confirm the reduction effects of humic substances on inhibitory effects of lead and cadmium on urease and acid phosphatase activity in this research. Keywords : Humic acid, Leonardite, Soil biological properties, enzyme activity, toxic metals.