Chitosan is a natural and destructive polymer that consists of glucosamine units, which due to reactive groups, can reduce the mobility of heavy metals. Pollution of heavy metals in soil and agricultural products is one of the most important environmental issues that threatens the life of plants, animals and humans. Soil enzymes are sensitive indicators that can well reflect the effects of different management practices on the soil. Little information is available about simultaneous effect of chitosan and heavy metals on soil enzymes. In this study, interaction between chitosan and heavy metals on the activity of some soil enzymes was investigated. Heavy metals including lead and cadmium were added to the two soils, Langroud and Lavark, at three levels of 0, 50 and 500 mg/kg. The soils containing heavy metals as well as the control soils were incubated for one month at 25 °C under 50% water holding capacity. Following the incubation, the soils were treated by chitosan at three levels of control (C), low molecular weight (LMC), and high molecular weight (HMC) and incubated for 15 days under previous conditions. Amidohydrolase and phosphomonoesterases activities including urease, L-asparaginase, L-glutaminase, and acid and alkaline phosphatase were measured. Fluorescein diacetate hydrolysis was also measured. In Langerode soil, HMC application to cadmium and lead treatment increased acid phosphatase activity and decreased alkaline phosphatase, urease, L-glutaminase, and L-asparaginase activity and fluorescein-diacetate hydrolysis. LMC application to cadmium treatments also led to an increase in urease, L-glutaminase, and acid and alkaline phosphatase activity and a decrease in L-asparaginase activity and fluorescein-diacetate hydrolysis. Application of LMC to lead treatments increased activity of urease, L-asparaginase and L-glutaminase and decreased acid phosphatase activity and hydrolysis of fluorescein-diacetate. In Laverk soil, HMC presence increased the activity of urease, L-asparaginase, L-glutaminase, and acid phosphatase and fluorescein-diacetate hydrolysis in cadmium and lead treatments in comparison with control, while activity of alkaline phosphatase increased due to HMC application in lead treatments and increased in cadmium treatments. In addition, LMC application to cadmium and lead treatments increased urease, L-asparaginase, L-glutaminase, acid phosphatase, and alkaline phosphatase activity and fluorescein-diacetate hydrolysis in Lavark soil. Significant positive correlations were observed among the soil bio-indicators. Whereas, the bio-indicators and the heavy metal concentrations were negatively associated. Increasing or decreasing enzyme activity due to the simultaneous presence of chitosan and heavy metal depended on soil type, soil enzymes, type of metal, chitosan, heavy metal level. Assay of soil enzymes as soil biological indicators can be well used to demonstrate the effects of amendments on soil in heavy metal contaminated soils. Key Words Chitosan, Amidohydrolases, Phosphomonostrases, Fluorescein-diacetate hydrolysis, Cadmium, Lead