The oxygen-transfer efficiencies or bubble size distributions of polyurethane membrane diffusers are closely related to the physical properties of the membranes and the operating conditions. The factors affecting the bubble sizes and pressure losses of micro perforated polyurethane membrane diffusers were investigated. The Sauter mean diameter and bubble size distribution of a diffuser depended on the needle diameter and type of polyurethane membrane. The Sauter mean diameter of an H-90 membrane was 1.33 mm which was smaller than those for other types of membrane. The pressures drop across the membrane depended on the immersion period in water and increased with soaking time. Changes in the Young's modulus after water soaking and the hardness of the membrane are assumed to cause the pressure drop across the membrane.