Water-borne polyurethane with branched straight aliphatic chains (CWPU) was prepared and characterized by FTIR and TEM. The relationship between structure and water-resistance of CWPU was studied. The results showed that the particle size of CWPU emulsion increases and the round particle turns into the spindle-shaped particle with increasing of content of branched straight aliphatic chains. However, the particle size of CWPU emulsion is not sensitive to the increases of length of branched straight aliphatic chains. The water-resistance and hydrophobic property of CWPU film increase with the increase of content and length of branched straight aliphatic chains. It is attributed to the hydrophobic layer of aliphatic chains enriched on the surface of CWPU film. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 1536-1542, 2011