A theoretical model for mean bubble size and frequency based on the collision theory with random spatial bubble distribution in freely bubbling gas-fluidized beds was developed. A hemispherical bubble velocity diagram was constructed to determine the average bubble collision frequency. The resulting theoretical equation for predicting bubble size is given. Bubble characteristics such as pierced bubble length, velocity and point frequency were measured in a square (30 multiplied by 30-cm) fluidized bed by means of an electroresistivity probe. The gradient of bubble size increased linearly with bubble voidage. The bubble Froude number also increased along the bed height with bubble voidage. The bubble Froude number represents approximately a linear relationship with the average fractional change of square root of the static energy of bubble rise along the bed height.