To develop a novel microporous polyurethane (PU) membrane that has higher water vapor transmission rate (WVTR) for use in functional application, down powder with different average particle sizes was blended with PU by using the solution casting method. Scanning electron microscope was used to investigate the changes in the porous structures of PU/superfine down powder (SDP) composite membranes. Attenuated total reflectance Fourier transform infrared spectroscopy curves showed that the addition of SDP decreased the hydrogen bonds of PU matrix. On the basis of wide-angle X-ray diffraction curves, the small crystallinity of the PU matrix in PU/SDP composite membranes was lower than that of pure PU membrane. The densities of PU/SDP composite membranes decreased with the increase in SDP content. PU/SDP composite membranes showed a large increase in WVTR and a small decrease in mechanical properties compared to pure PU membrane. Moreover, the WVTR and mechanical properties of PU/down powder composite membranes were found absolutely dependent on the average particle size of down powder. POLYM. ENG. SCI., 50:2400-2407, 2010. (C) 2010 Society of Plastics Engineers