The phase separation of ultrahigh molecular weight isotactic polypropylene (it-UHMWPP) solution in the gelation process was investigated by quenching the solutions to the desired temperature. When an incident beam of an He-Ne gas laser was directed to the gel, the logarithm of scattered intensity increased linearly with time in the initial stage and tend to deviate from this linear relationship in the later stage. The melting endotherm. of the differential scanning calorimetry curve showed a clear peak in the later stage, but no peak could be observed in the initial stage, indicating poor ordering of molecules in polymer-rich phase. The scattered intensity of laser beam from the gel showed a peak in the scattering angle direction. The peak became more intense with increasing time, but the peak position did not change. Accordingly, the phase separation of UHMWPP solution in the initial stage was analyzed within the framework of the linear theory of spinodal decomposition. In parallel to this small-angle light-scattering experiment, the time-resolved measurements in the gelation process were carried out for X-ray diffraction and infrared and Raman spectra. These data could be interpreted reasonably in terms of formation of crystallites as cross-linkages of the gels. Under optical microscopy, it was confirmed that the gels prepared by quenching the solution to room temperature compose of periodic honeycomblike structure characterizing the spinodal decomposition of the solution due to thermodynamic instability. The average size of this periodic structure is slightly larger than that estimated from the scattering angle to give the maximum value of growth rate of concentration fluctuation. Periodic honeycomblike structure was also observed for the dry film under a scanning electron microscopy. The hole size of the honeycomblike structure became smaller as the quenching temperature decreased, and the size distribution was narrow. The dried films were stretched up to 60 times, as has been reported already. The possibility of successful elongation up to lambda = 60 became higher with decreasing quenching temperature. Young's modulus, crystallinity, and molecular orientational degree of the resultant dry gel films became higher with decreasing hole size of honeycomblike structure. Accordingly, it turns out that the dense network structure created by the spinodal decomposition of the solution plays an important role in ensuring smooth transmission of inner stress in the stretching direction and ultradrawing of UHMWPP film can be achieved.