Abstract: Bubble formation frequencies in a single-orifice two-dimensional gas-solid fluidized bed(292mm × 16mm) were measured by means of a high sensitivity capacitance probe. A spectrum analyzer was used to analyze the bubble frequency distribution. The effects of various parameters on the bubble frequency power spectrum density were investigated. The following parameters were included: particle size(0.105-0.590mm), particle density (590-2990kg/m3), minimum fluidization velocity of particles (0.0072-0.481m/s), initial bed height (205-565mm), distance of probe above orifice, flow rate at orifice (0.5-35 × 10-4m3/s) and background fluidizing gas flowrate (0-3 times of minimun fluidization velocity). For a fluidized bed with small particles of low densities, the bubble formation frequency conformed very well with the Davidson and Harrison model. A regular deviation from the model was observed with increases in particle sizes and densities. A modified model taken into account, for the gas leakage from forming bubbles into the surrounding emulsion phase, when dealing with larger particles, was presented. The modified leakage model proved to agree with the experimental results obtained.