Semicrystalline polymer (e.g., polyethylene and polypropylene) particles may swell in solvent at elevated temperatures, inducing flow instability in operation, reactor blockage, and undesired product. Understanding the fluidization behavior of swelling particles at elevated temperatures is therefore of great significance. This work conducts an experimental study on the fluidization of polyethylene particles in an elevated-temperature liquid−solid fluidized bed. With the increase in temperature, particle swelling develops over two phases, i.e., non-cohesive swelling and cohesive swelling, which is different from the fluidization of cohesive particles in gas−solid systems. Four regimes can be identified through static bed height and pressure drop measurements, i.e., stable fluidization, noncohesive-swelling fluidization, cohesiveswelling fluidization, and full defluidization. A phase diagram was then obtained to demarcate these regimes. Furthermore, an energy balance model was proposed to predict the initial agglomerating temperature T ag of swelling particles for early warning of abnormal fluidization in practical operation.