Humic acid as an organic substance can bound heavy metals. It is a best way of decreasing the hazardous effect of heavy metals on plants and thereby on humans. A factorial experiment was conducted in a Completely Randomized Design of seven replications. The treatments consisted of three cadmium levels (0, 2 and 4 mgL-1), and three humic acid levels (0, 100, 1000 mgL-1) added to nutrient solution. Superoxide dismutase and peroxidase activities, biomass and leaf cadmium contents in lettuce were assessed. Peroxidase activity increased up to 7 and 29% and superoxide dismutase to 39 and 19% with the addition of 2 and 4 mgL-1 of cadmium respectively. When humic acid was added, the activity of antioxidant as well as the reduction in biomass content became more moderated. Leaf cadmium content increased with increasing cadmium concentration in the nutrient solution, whereas it decreased when humic acid was added to the nutrient solution.