In-line particle size measurement with a spatial filter velocimetry probe for process control was already carried out successfully in conventional fluidized bed coating and agglomeration but has not yet been investigated in rotary fluidized bed agglomeration with a tangential spray for granulate and pellet manufacturing. In this project, a batch laboratory fluidized bed apparatus with a rotor insert and a tangential spray nozzle was used to investigate the feasibility of in-line particle size measurement with a spatial filter velocimetry probe and to detect critical process parameters. First, particle size and particle size distribution of a placebo mixture including small and large pellets were measured directly with the in-line probe in the rotary fluidized bed without spraying and without agglomeration. The volume density distribution shows bimodal curves, the peaks of which are related to the different ratio of pellets. Second, microcrystalline cellulose powder was agglomerated with a binder solution in the rotary fluidized bed by tangential spraying. At-line dynamic image analysis for the particle size measurement was used for comparison and showed similar results to the spatial filter velocimetry method. A Plackett-Burman screening design identified spray rate, atomizing air pressure, rotor disk velocity and batch size as critical process parameters by measuring particle size, particle size distribution and sphericity. The spatial filter velocimetry probe can be used in rotary fluidized bed agglomeration with tangential spray for process control. (C) 2020 Elsevier B.V. All rights reserved.