The absolute and differential pressure fluctuations in gas-solid fluidized beds have been analyzed by statistical and deterministic chaos methods. Linear low-density polyethylene (LLDPE) particles with a mean diameter of 1.23 mm were used as a fluidizing material. The statistical methods are composed of the mean, standard deviation, skewness and kurtosis, and the deterministic methods are composed of autocorrelation, mutual information function, pseudo-phase space and correlation dimension. The minimum slug velocity of LLDPE particles is found to be 0.34 m/s by using the statistical and deterministic methods. As slugs appear and grow with increasing gas velocity, pressure fluctuations in the fluidized bed of LLDPE are oscillated and more periodic.