Abstract: To explore low-dose hormetic effects (a biphasic dose-response characterized by a low dose benefit and a high dose inhibition) of Cd²⁺ and Cr³⁺on activity of alkaline phosphatase in wetland soil, soil samples were collected from the wetland in Dongtan of Chongming, treated with different doses of Cd²⁺ and Cr³⁺, making the samples 0, 0.001, 0.01, 0.1, 1, 5, 10, 20, 100 and 500 mg·kg^-1 in Cd²⁺ concentration and 0, 0.5, 5, 50100, 500 and 5000 mg·kg^-1 in Cr³⁺ concentration, separately, and then incubated for 0, 6, 12, 24, 48, 72 and 120 hrs. After the incubation soil samples were analyzed for activity of alkaline phosphatase (ALP)in soils to determine characteristics of the temporal variation of the activity with the incubation. Results show that after 12 hours of incubation, ALP activity was 8.6% higher in the soil samples 1 mg·kg^-1 in Cd²⁺ concentration than in CK, and was obviously inhibited in the soil samples 10 mg·kg^-1 in Cd²⁺ concentration(P<0.05), while ALP activity was 22.8% higher in the soil samples 5 mg·kg^-1 in Cr³⁺ concentration than in CK (P<0.05), but substantially lower in the soil samples 100 mg·kg^-1 or higher in Cd²⁺ concentration, which indicates that the two types of heavy metals have some low-dose hormetic effects on ALP activity in the wetland soil, and the effects were closely related to duration of their contact. Meanwhile, after 24 hours of incubation, V_max/K_m (maximum reaction velocity/Michaelis constant) was 1.7 in the soil samples 1 and 5 mg·kg^-1 in Cd²⁺ concentration and lowered down to 0.8 in the soil samples 20.0 mg·kg^-1 in Cd²⁺ concentration with both V_max and K_m being lower than their respective ones in CK (soil samples 0 mg·kg^-1 in Cd²⁺ concentration), and V_max/K_m was also 1.7 in the soil samples 1 and 5 mg·kg^-1 in Cr³⁺ concentration, and 1.4 in the soil samples 100 mg·kg^-1 in Cr³⁺ concentration, with both V_max and K_m being higher than their respective ones in CK. All the findings indicate that both V_max and K_m fall simultaneously in the presence of Cd²⁺, but rise simultaneously in the presence of Cr³⁺, and that the mechanism of low-case hormesis existing between heavy metals and soil enzymes may be closely related to properties of the heavy metal ions.