The catalytic reactions of ethylene and hydrogen have been investigated in a fluidized-bed reactor containing Ni nanoparticles and microsize SiO2 particles. Although significant amounts of carbon were formed on the Ni surface, the Ni catalyst was not completely deactivated. However, the formation of carbon could lead to defluidization. Increasing temperature from 665 to 800 A?C drastically improved the fluidization. The catalytic activity of the Ni catalyst changed with reaction time, showing a maximum at 750 and 800 A?C. The crystalline size of Ni as determined by X-ray diffraction also decreased with reaction time. Although the dehydrogenation of ethylene was the dominant reaction initially, the hydrogenation of ethylene to ethane became increasingly important with time.