The effects of secondary air ratio (0-50 %), height of injection port (1.5-2.5 m) and injection types (radial and tangential) on the axial solid holdup distribution have been determined in a circulating fulidized bed (0.1 m I.D. x 5.3 m high). As the secondary air ratio is increased, solid holdup increases below the secondary air port, whereas it decreases slightly above the secondary air injection port. The dense bed height increases with increasing height of the secondary air injection port. Different types of secondary air injection system do not produce any significant difference in the overall axial solid holdup distribution. However, solid holdup just above the injection port in a tangential secondary air injection system exhibits somewhat higher values than those of radial secondary air injection system. The effect of secondary air injection on the axial solid holdup distribution in a circulating fluidized bed can be predicted by a modified freeboard-entrainment model with decay constants determined from the proposed correlation.