Bubble flow characteristics were investigated in gas-solid bubbling fluidized beds (0.3 m-I.D×2.4m-high) with or without internals by power spectral analysis of absolute pressure fluctuation. Metallurgical grade silicon particles (MG-Si) were used as bed materials. The particle density and mean particle diameter were 2,328 kg/m3 and 154 μm, respectively. Absolute pressure fluctuations were measured simultaneously at two different positions: plenum chamber and beds. Absolute pressure fluctuation in the beds was measured according to the axial bed height in the range of 0.1 to 0.8m. The total sampling time of each data set was 60 s, and the sampling rate was 200Hz. Absolute pressure fluctuation data were converted to a power spectral density (PSD) by a Fast-Fourier transform (FTT) algorithm. The PSD in the beds was separated into coherent and incoherent output power. The bubble size was estimated from the standard deviation of the spectrum of incoherent output power, which occurred due to the bubble flow. The estimated bubble size determined by incoherence component analysis was compared to various empirical correlations to determine the bubble size without internals. The estimated bubble size agreed well with the correlation by Choi et al. [19]. The internals were installed 0.45 m above the distributor. With the installation of the internals, and at the bed height of 0.5m, the bubble diameter was decreased by 77% compared to the bubble without the internal at U0=0.15m/s.