The pressure fluctuations in a Multi-Solid Fluidized Bed (MSFB) are investigated under various experimental conditions with a lab-scale MSFB and a pilot-scale MSFB. The main purpose of the studies is to show the effect of coarse particles on pressure fluctuations in MSFBs and to identify the MSFB operating mode using power spectrum analysis. The dominant frequencies of pressure fluctuations are also determined under various experimental conditions. The pressure fluctuations show the difference in coarse particle bed behavior depending on whether fine particles are circulated or not. The power spectra of CFBs change qualitatively in the presence of a dense bed of coarse particles. If a sharp peak is observed in the power spectrum, it means that a dense bed of coarse particles exists in the bottom part of the riser; otherwise, coarse particles are dispersed in the riser and/or circulate in the reactor.