The in-flight heating process of powder particles in radio frequency (RF) thermal plasma has great importance for plasma spraying and spheroidization. The present work investigated this in-flight heating process of cerium dioxide (CeO2) powders, both experimentally and numerically. In the experiment, commercial CeO2 powders (30 μm avg.) were injected into the RF argon plasma and temperatures were measured using a DPV-2000 monitor. A model that combines the electromagnetic, thermal flow, and heat transfer characteristics of powder in-flight heating in argon plasma was proposed. The melting process of the CeO2 powders with different diameters, as well as the melting time of powders under thermal resistance effects, was investigated. It was found that the heating process of powder particles consists of three main stages, one of which the heating stage is relevant to a dimensionless parameter known as the Biot number. When Biot≥0.1, thermal resistance increases significantly, especially for larger powders. The predicted temperature of the particles at the outlet (1800-2880K) is in good agreement with the experimental results.