Biodenitrogenation of petroleum oil was investigated by a previously isolated carbazole-degrader Pseudomonas sp. XLDN4-9. In a tetradecane-aqueous phase system, biodegradation of carbazole was enhanced by the presence of n-tetradecane. And strain XLDN4-9 was capable of absorbing 95.2% of 2 g/L carbazole dissolved in diesel within 15 hours. Significant denitrogentation of crude oil, diesel and lubricanting oil was detected by strain XLDN4-9. Removal of carbazole, methylcarbazole, and dimethylcarbazole in diesel was confirmed by using GC-MS. After 3 days, 99% of carbazole and 15% of dimethyl carbazole was degraded. And the removal rate of 1-, 2-, 3-, and 4-methyl carbazole was determined to be 63.4%, 87.6%, 78.4%, and 66.5% respectively.