In order to understand the slugging regime comprehensively, the characteristics of slugging have been studied experimentally in gas fluidized beds. The onset velocity of slugging and the properties of slug were measured in gas fluidized beds(bed A: 0.055 m i.d., 2.0 m height, bed B: 0.1 m i.d. and 2.5 m height) using sand(specific surface mean diameter: 0.286 mm) and FCC(FCC A, specific surface mean diameter: 0.082 mm, FCC B, specific surface mean diameter: 0.091 mm) as bed materials with variations of gas velocity, height from the distributor plate, bed height at minimum fluidization, column diameter and particle properties. The minimum slugging velocity decreased with increasing height from the distributor plate, however, increased with increasing bed height at minimum fluidization and column diameter. However, the effect of particle properties was found to be dependent on the column diameter. The minimum slugging velocity of sand was higher than that of FCC in the column of 0.055 m inside diameter, however, that of FCC higher than that of sand in the column of 0.10 m inside diameter. The slug frequency decreased with increasing gas velocity, height from the distributor plate and bed height at minimum fluidization, however, increased with column diameter. The effect of gas velocity on slug frequency diminished as gas velocity increased. The effect of height from the distributor plate on slug properties decreased as the bed height at minimum fluidization increased. The slug frequency of FCC was higher than that of sand at the same excess gas velocity.