Pressure fluctuations and transient flow behaviors of particles were investigated in a gas-solid fluidized bed of 0.105-m ID and 2.5-m height. The pressure fluctuations in the beds at the transient state as well as at steady state were measured and analyzed by adopting the chaos analysis method. We found that the transient flow behavior of particles in the bed could be classified into two states, heterogeneous and homogeneous expansion states, before the flow behavior attained a new steady state condition. The change of particle flow behavior could be detected from analyses of the phase space portraits and the correlation dimension of the pressure fluctuations in the bed. In addition, the holdup change of the solid particles in the test section was obtained from the time series of pressure fluctuations at a transient state.