The local behavior of gas and solids in a grid zone was investigated using a jetting fluidized bed cold model. Spherical alumina particles (440 mum, 1500 kg/m3) were used as bed material. A pair of optical fiber sensors, an A/D converter and a personal computer were used to measure particle velocity in the jet, jet diameter and jet height. Tracer gas introduced from a center nozzle was sampled from various points in the grid zone and its concentration was measured by gas chromatography to investigate gas interchange between the jet and the annulus, and the gas dispersion in the annulus region. The effects of operational conditions, i.e., the shape of the gas distributor, nozzle diameter and gas flow rates, on the gas and particle behavior are discussed. It was considered that many more particles entered into the jet when a cone-shaped distributor rather than a flat distributor was employed. Employment of a narrower nozzle caused an increase in particle velocity at the bottom of the bed.