Chemorheology of neat polyurethane (PU) system, composed of a diisocyanate and a polyol, and a PU/clay nanocomposite system with an organoclay 5 phr was investigated. Differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) were used to study the polymerization behavior of both systems. The PU/clay nanocomposite system showed a faster polymerization rate than neat PU system. The rheological property change during isothermal and dynamic polymerization of both systems was measured using a rheometer to analyze chemorheology of them. A viscosity function that can describe viscosity change of a thermoset resin system during polymerization was adopted to analyze the viscosity data obtained from the rheometer. The PU/clay nanocomposite system had shorter gelation time than neat PU system. Structural analyses of the PU/clay nanocomposite were performed by X-ray diffractometer (XRD) and transmission electronic microscope (TEM).