Microbial treatment of diesel was carried out with the strain Pseudomonas aeruginosa OCD 1 in liquid Bushnell-Haas broth to study the changes in physico-chemical properties of treated diesel. The optimum culture conditions for this strain were obtained as temperature 30°C, shaker speed 125 r.p.m and pH 6 in liquid Bushnell-Haas broth with 2% diesel as a substrate. The evaluation of physico-chemical properties reveals that the percentage of alkanes are reduced in the treated diesel as aniline point, diesel index, cetane number, pour point values were decreased for treated diesel compared to untreated diesel. This strain therefore can be used for the quality improvement of petroleum products with respect to pour point or for the purpose of dewaxing of lubricating oil base stock (LOBS).