The gelation process and the microstructure after the gelation of polyurethane resin consisting of acrylpolyol and polyisocyanate was investigated by dynamic light scattering (DLS) as a function of stoichiometric ratio, [NCO]/[OH], and the concentration of acryl-polyol, C-polyol. The following conclusions were obtained. First, the sol-gel transition was explained by the so-called site-bond percolation theory. Here polyol groups act as site-occupants, and isocyanate groups act as chemical cross-linkers. Second, the scattering inhomogeneities increased rather gradually around the gelation point, which are the characteristics of the gelation process not from monomeric but from oligomer units. Third, at the gelation point, the characteristic decay time of the fast mode, tau(fast), decreased, and the fraction of the collective diffusion mode, A, increased with [NCO]/[OH], which are due to introduction of cross-linking and/or increase of the rigidity of the network. Finally, the ensemble average scattering intensities, < I >(E), the static inhomogeneities, < I-C >(E), the time-average dynamic fluctuating component, < I-F >(T) and the collective diffusion coefficient, D, showed remarkable dependence not only on C-polyol but also on [NCO]/[OH]. These are due to the competition between the cross-linking and the progress of micro-phase separation. (C) 2009 Elsevier Ltd. All rights reserved.