Three-dimensional computational fluid dynamics (CFD) simulations of hydrocarbon fireballs have been performed for their detailed characterization, including diameter, lifetime, flame, and internal fireball structure. The diameter and duration of a fireball obtained by CFD simulations and available empirical models have been compared with experiments and video footage data. CFD simulations are essential as empirical models have inherent assumptions and limitation in describing internal fireball structures. The fireball resulting from the loss of a fuel tank exposed to fire (experiment conducted by the German Bundesanstalt fur Materialforschung in 1999) and a fireball from an aircraft crash (aircraft ambulance crash in Iceland in 2013) are considered for the CFD analysis. Influence of initial momentum, turbulence, and radiation on fireball structure has been analyzed for both of the cases. To illustrate transient behavior, developed pressure and flame structures are studied during the evolution of a fireball. Fireball diameter and lifting height computed from numerical analysis is found to be in good agreement with available video footage data. The incident radiations on the ground are calculated using a numerical method along the radial position from the center of the fireball, to predict the thermal hazards from the fireball.