We report the measurements of dynamic light scattering (DLS) and dynamic viscoelasticity on an unsaturated polyester/styrene system at various degrees of curing through sol-gel transition. The measurements have been carried out with both polarized and depolarized scattered light. The degrees of conversion at various curing stages were measured by differential scanning calorimetry (DSC). The experimental results indicated that the depolarized DLS correlation functions were independent of the degree of curing. The polarized DLS data revealed two modes of relaxation before gelation but only one mode of relaxation after gelation. The correlation function of polarized DLS varied with degrees of curing and approached that of depolarized DLS after gelation. Both the DLS and viscoelastic measurements showed the divergence of relaxation times at gelation, indicating the consistency of these two measurements for defining gelation.