This dissertation investigates the sintering of Ultra-High-Molecular-Weight Polyethylene (UHMWPE) using in-situ techniques. Despite its widespread use, the manufacturing process of UHMWPE is not fully understood. Specifically, the short processing time under moderate pressure contradicts analytical models predicting particle coalescence and interfacial strength buildup, given its low surface energy and high viscosity. This research represents one of the first systematic studies dedicated to qualitatively identifying the macroscopic volume change during the overall sintering process of nascent UHMWPE powder. The goal is to monitor and reveal deformation during the manufacturing process, ultimately for a better understanding of the structure-process-property relationships of UHMWPE. The study begins with pressure-free sintering of UHMWPE nascent powder to investigate the influence of compaction pressure on the subsequent deformation of the sintering stage. Without pressure during sintering, significant expansion is observed during heating through the α-relaxation and melting. This large expansion impedes the porosity removal during isothermal sintering, therefore leading to high porosity remaining in the sintered UHMWPE and insufficient properties for applications. Since pressure is essential for porosity removal, a customized pressure sintering apparatus are developed, providing in-situ density evolution. Specifically, five distinct processes are identified including: (1) room temperature compaction; (2) subsequent densification through the α-relaxation, (3) enthalpy-driven melt explosion via crystal melting; (4) entropy-driven melt explosion due to non-equilibrium melt; (5) recrystallization under pressure. Thus, this in-situ density is applied to study varying external processing parameters and molecular architecture. The mechanical properties of sintered UHMWPE are evaluated, focusing on impact behaviors and using fracture mechanics to compare crack resistance under severe conditions. Both metallocene-catalyzed- and Ziegler-Natta-catalyzed- UHMWPE exhibit ductile fracture behaviors with significant plastic deformation, evidenced by fibrils observed through microscopy. Additionally, higher molecular weight reduces diffusion, leading to weak interface and the formation of grain boundaries. Finally, the blends of UHMWPE-HDPE is studied aiming to enhance the processibility. Interesting results are observed with a mass concentration of 20% UHMWPE in the blends. Preliminary results indicate that 20% UHMWPE can enhance load transfer ability while maintaining higher crystallinity.