Abstarct: Petroleum wastewater contains large number of hydrocarbons and low biodegradable compounds which make its treatment serious challenge in field of environmental engineering. Biological process needs lots of time and leads to sludge production than require extra treatment. As a result, biological process cannot be good enough considering volume of oil effluent production in these days. Ozonation as an advanced oxidation process with powerful oxidizing agent along with micro bubbles has investigated in this study introducing a new innovation in the petroleum wastewater treatment process. At end of study, COD removal efficiency in micro bubble reactor was about 91% which was 34% higher compare to COD removal in macro bubble reactor. 60% of alkanes has removed from MIBR while it just increased in MABR. Benzene, ethyl-benzene, toluene, xylene (BTEX) had removal efficiency of 14%, 83%, 20% and 87% respectively in MIBR and 7%, 34%, 20 and 47% in MABR. TPH has increased in first 8 hour and started to decline in MIBR while, in MBR it had an increasing trend. Results of this study cleared that micro bubble ozonation has better function in removal of COD, BTEX and alkanes in comparison of macro bubble ozonation which could introduce this method as a pre-treatment method in petroleum wastewater treatment plant.