The gelation mechanism of polyethylene solutions was investigated by using light scattering and X-ray diffraction techniques in terms of the liquid-liquid phase separation. Three kinds of polyethylene, ultrahigh molecular weight polyethylene (UHMWPE), low molecular weight linear polyethylene (L-LMWPE), and low molecular weight branched polyethylene (B-LMWPE), were used as test specimens. When an incident beam of He-Ne gas laser was directed to the UHMWPE and B-LMWPE solutions quenched to a desired temperature, the logarithm of scattered intensity increased linearly in the initial stage and tended to deviate from this linear relationship in the latter stage. If the linear increase in the initial stage can be analyzed within the framework of the linear theory of spinodal decomposition proposed by Cahn, the gelation is obviously attributed to the phase separation leading to the concentration fluctuation of the solution. Furthermore, in the later stage showing the deviation from the linear relationship, light scattering from the UHMWPE solution with 0.5% concentration under Hv polarization condition showed an X-type pattern, indicating the appearance of optically anisotropic rods. On the other hand, the scattering from the B-LMWPE solution with 3% concentration showed a four-leaf clover pattern, indicating the appearance of optically anisotropic spherulites. No crystallite was confirmed by the X-ray diffraction measurements, when the rods and spherulites appeared. With further lapse of time, the Hv patterns became clearer and the corresponding X-ray diffraction intensity curves showed a very small diffraction peak from the (110) plane. In contrast, for the L-LMWPE solutions, the logarithm of scattered intensity against time showed a rapid increase and tended to level off because of rapid change from slightly transparent to whiten gels. In such a process, any quantitative analysis of the scattered light intensity was impossible. The corresponding X-ray diffraction revealed the strong reflections from the (110) and (200) planes, indicating the rapid formation of stable crystallites. Through a series of experiments for the three kinds of polyethylene solution, it turned out that gelation mechanism of polyethylene solutions is strongly affected by molecular weight and the degree of branching.