We studied effects of free copper ion activity and total copper concentration on copper uptake by lettuce from nutrient solution and a model was developed. In ethylenediaminetetraacetic acid and dissolved organic matter treatments, when pH and free copper ion activity were the same, root copper concentration did not change with the total copper concentration. However, at fixed pH and total copper concentration, root copper concentration increased with, and was log-log linearly related to free copper ion activity. Copper uptake by plant roots is affected by free copper ion activity, pH, and the concentration of other competing ions such as Ca2+. Based on the free-ion activity model, we developed a model to predict copper concentrations in lettuce root and shoot. The parameters in this model are the binding constant of lettuce root surface site for copper times the site density, log(nKCu) = 11.29 ± 0.10; for protons, log(KH) = 6.80 ± 0.12; and for calcium, log(KCa) = 8.49 ± 0.30. Copper concentration in lettuce shoot was much lower than that of lettuce root. The translocation coefficient (k) is (2.36 ± 0.37) × 10−2.