The gas discharge modes, bubbling and jetting, formed at a single horizontal nozzle in a two-dimensional gas fluidized bed were investigated by visual observation. The inside diameters of the nozzles used in this study were 3.0, 4.5, 6.0, 7.5 and 9.0 mm. The gas velocity through the nozzle was in the range from 1 to 130 m/s. The ratio of the superficial velocity to the minimum fluidizing velocity was in the range from 0 to 4. Gas discharge modes were analyzed by high speed cine photography, and the effects of the nozzle diameter, gas velocity through the nozzle, particle diameter and the static bed height were studied. The results showed that the length of jet penetration increased with the gas velocity through the nozzle and the nozzle diameter. The phase diagram of bubbling, jetting and transition zones were mapped by using a modified Froude number and the ratio of the nozzle diameter, D-o, to the particle size, D-p.