The horizontal rotary retort is one of the important equipment in oil shale retorting by solid heat carrier technology, in which the heating of oil shale plays a crucial role in shale oil production. In this work, a comprehensive heat transfer model including both conduction and radiation under contact and non-contact conditions was proposed to numerically investigate the heat exchange among particles at five different pathways by employing the discrete element method (DEM). The effects of particle physical parameter (particle size) and operating conditions (rotational speed and fill ratio) on thermal characteristics such as oil shale heating time, thermal rate, thermal index and heat flow were deeply analyzed. In summary, the heat flow generated by particle-fluid-partide heat conduction contributed 66-71% of the total heat flow, the radiative flow produced by particle-particle heat radiation was 22-30%, and the thermal contribution of particle-particle heat conduction was the least, accounting for only 6-7%. (C) 2020 Elsevier B.V. All rights reserved.