The effect of urea on the aggregation behavior of proanthocyanidin polymer from Pinus radiata (P.r.) extract has been studied by rheological measurements and photon correlation spectroscopy (PCS). The urea induced structural changes in the concentrated extracts that were accompanied by significant reductions of apparent viscosity. The size reduction of proanthocyanidin polymers upon addition of increasing amount of urea could also be observed by PCS. In a concentrated alkali-sequential extract, addition of urea leads to a decrease in both the steady shear viscosity and the dynamic elastic modulus. Since the urea-induced structural changes of proanthocyanidin polymers are based mainly on the intermolecular colloidal association, the overall variation of reduced viscosity in the low concentration range of proanthocyanidin in the presence of various amount of urea was negligible. Thus it is suggested that addition of urea leads to the control of the noncovalent colloidal interactions among proanthocyanidin polymers from Pinus radiata. In the absence of urea, concentration of the P.r. extract to 40% tannin produced stable near-Newtonian solutions of low apparent viscosity.