The hydrodynamic velocity and effect of the reaction temperature on the cracking performance Were investigated for the production of light oil through the rapid pyrolysis of heavy oil in a circulating fluidized bed reactor. From this study, the gas velocity in the loop seal to prevent slugging, the transport gas velocity to form a dense phase of solids on the bottom Of the -" riser, and the minimum bubbling fluidization velocity for stable solid circulation in the reheater were determined. Under the steady supply of feed through the internal mixing with steam inside the feed nozzle, the reactivity for a terriperattire change from 527 to 574 degrees C was investigated. As a result, the residue (>535 degrees C) conversion and liquid yield of up to 71.1 and 78.2%, respectively, were found. The maximum impurity rernoval rates for sulfur, Conradson carbon residue, and metals (nickel and vanadium) were found to be 1.8.7, 50.4, and 83.6%, respectively.