The effects of particle morphology on the structure and swelling/dissolution and rheological properties of nascent ultra-high molecular weight polyethylene (UHMWPE) in liquid paraffin (LP) were elaborately explored in this article. Nascent UHMWPE with different particle morphologies was prepared via pre-polymerization technique and direct polymerization. The melting temperature and crystallinity of UHMWPE resins with different particle morphologies were compared, and a schematic diagram was proposed to illustrate the mechanism of UHMWPE particle growth synthesized by pre-polymerization method and direct polymerization. The polymer globules in the nascent UHMWPE prepared by using pre-polymerization technique are densely packed and a positive correlation between the particle size and the viscosity-averaged molecular weight can be observed. The split phenomenon of particles and the fluctuation in the viscosity of UHMWPE/LP system prepared by direct polymerization can be observed at a low heating rate and there is no correlation between particle size and viscosity-averaged molecular weight.