Release property as well as release mechanism of drug loaded in polymer membrane undergoing swelling were studied. Release patterns of PEG from the dried membranes were evaluated for a series of molecular weight of PEG. The release patterns were quantified using the exponent n defined by the equation, M-t/M infinity=kt(n). The characterization (pore structure) of the membrane was also carried out by means of water permeation experiment. The membranes containing more than 90% water (wet basis) were found to form a pore structure available to promote penetrant convection in their swelling stage. Based on these experimental results, theoretical model of the swelling controlled release system was proposed. The effects of the structure of membrane, molecular size of drug and convection of penetrant on the release pattern were simulated well by this model. The mechanism of various release patterns was discussed from a viewpoint of diffusion of drug and relaxation of polymer network.