Dibenzylidene sorbitol (DBS) was chosen as an in situ forming nucleating agent to study ultrahigh molecular weight polyethylene (UHMWPE)/liquid paraffin (LP) physical gel and complex phase separation. The experimental results indicated that DBS self-assembled into fibrils first and the solution became a physical gel before liquid liquid phase separation (LLPS) and crystallization during thermally induced phase separation (TIPS) of UHMWPE/LP/DBS solution. The temperature of DBS self-assembly and viscoelasticity of UHMWPE/LP/DBS gel show a strong dependence on DBS concentration, temperature, and time. By controlling the relative quenching depth and annealing time, the grow rate of the characteristic length showed a crossover from LLPS to crystallization, which was further justified by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). With decreasing temperature further, crystallization occurred with the aid of DBS fibrils. Those interactions were affected mutually, showed complex phase separation behavior. We proposed a new mechanism of "self-assembly assisted liquid liquid phase separation", which explained excellently the relationship of DBS self-assembly and LLPS. On the basis of rheometer and optical microscopy (OM and POM), we obtained the phase diagram of UHMWPE/LP/DBS ternary blends. Meanwhile, DSC indicated the overwhelming changes of crystallization kinetics after the varying LLPS in the double quenching procedure, which was consistent with the "fluctuation assisted nucleation" mechanism.