From a primary screening, 127 isolates of oil degrading bacteria were collected from the environment. Mixed culture of M14 was eventually obtained by the mean of sequential screening in Bushnell & Haas supplemented with 1% Tapis crude oil. Oil degrading activity was determined by a capillary gas chromatography. Growth of microorganism was measured by the plate count technique on Nutrient Agar medium. Two strains of 7 isolates in M14 mixed culture were found to be predominant and identified as Pseudomonas aeruginosa (M14-01) and Acinetobacter baumanii (M14-07). To improve the efficiency of the crude oil biodegradation, the optimum parameters were pH8, 0.5% Tapis crude oil, 2.5 g/l nitrogen, 0.5 g/l phosphorus, 30 0C. In such optimum condition, total and C9-C33 hydrocarbons were degraded approximately 93.5% and 99.5%, respectively on day 4. Bioremediation of wastewater taken from Bangchak oil refinery by seeding (M14-01 and M14-07 mixed culture) and nutrient addition (2.5 g/l N and 0.5 g/l P) improved the efficiency of oil degradation, in comparison with either seeding or nutrient addition alone or the control (no seeding, no nutrient addition). In addition, the investigation of the biodegradation of 0.1 g mixed hydrocarbons showed that alkanes were degraded in the following order: shorter straight chain, longer chain and branched chain.