Vacuum residue obtained from Tehran oil refinery vacuum distillation tower and used in a bubble column fermentor as the sole source of carbon and energy in minimal salts medium and different aeration applied. The best results for bacterial growth obtained at 0.08–0.3 v.v.m. Specific gravity of vacuum residue after biodegradation also decreased from 0.98 to 0.95. Decreasing specific gravity could be an indication for bio-lightening. Gas holdup was in the range of 0.007 to 0.062 and was well fitted with Hugmark correlation.