Pressure fluctuations were measured in a fluidized bed plenum and power spectral density functions obtained by fast Fourier transform (FFT) analysis of the pressure fluctuation and its confidence limit for the diagnosis of fluidization were investigated. Various species of fluidized beds, which consist of Groups A. B or D particles of Geldart's classification and their mixtures, were used. Species of fluidizing particles, weight fraction of particles in bed and bed height strongly influenced the mode of fluidization. They were also significantly reflected on the power spectral density function. From experimental results, it is concluded that frequency analysis of pressure fluctuation is useful to obtain information about fluidization and to diagnose the fluidized bed, while it has a limit for its ability for detection of the fluidization mode.