Fluidized bed is an important unit operation used in many industries for different applications, including drying, coating and granulation. When hot inlet air is used, the particles and the fluidized bed wall are heated in the same time. In order to understand the heating process of the particles, it is necessary to online monitor the particle temperature. Due to the influence of the air around the particles, contact devices (e.g. thermocouple probes) are not suitable for measuring the fluidizing particles. Therefore, thermal cameras were used recently to online monitor the temperature of the fluidizing particles non-invasively. In the literature, the particles in the thermal images were generally distinguished from the fluidized bed wall. However, due to the particle movement and the focus of the thermal camera, when using a thermal camera to monitor the particles in a cylindrical 3D fluidized bed, there will be clear particles and blurred particles in the thermal images. The blurred particles can affect the accuracy of the particle temperature measurement. In this paper, based on the size and shape of the particles, the clear particles in the thermal images were identified by filtering the fluidized bed wall and the blurred particles. Then the temperature measured from these identified particles were experimentally calibrated. Using this technique, the online temperature of the particles was successfully measured together with the online temperatures of the air and the fluidized bed wall. Crown Copyright (C) 2020 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers. All rights reserved.