This work uses a computational fluid dynamics approach for the simulation of a slurry bubble column reactor, applied in the hydroconversion of heavy oils under severe temperature and pressure operating conditions. Simulations are carried out in two steps : first, the fluid dynamics is determined by the calculation of the pressure drop in the bed and the radial distribution of gas and slurry for the holdup, effective viscosity (dispersion) and velocity. Then a thermal cracking reaction is simulated and the oil conversion to lighter fractions such as gasoil, diesel, naphtha and gases is obtained. Results show the recirculation pattern in the reactor which leads to a high degree of backmixing in the slurry phase. Temperature and liquid residence time are found to have a great influence in the oil cracking conversion.