Nitrate concentration above standard level if present in drinking water, possibly reduces to nitrite in the human digestive tract. Further converts hemoglobin to metheomoglobin with a great loss in oxygen-carrying capacity, which is responsible for developing metheomoglobina. The nitrite may react with amines forming carcinogenic nitrosamines. The wastewater of a petrochemical was initially treated with microorganism of the municipality of Shahrek-e-Ghareb located in Tehran. A prolonged, poor and irreproducible result was obtained, which was perhaps on the count of higher ratio of heterotrophic bacteria. Further literature experience the procedure as showing that the autotrophic organism synthesis nitrification of certain wastewater at higher rate than heterotrophic bacteria and fungi. The autotrophic bacteria from wastewater of depo 93 of Shiraz petrochemical plant was partially purified by dilution method through 13th generation. The 8th microbial consortium resulted as choice source of isolated bacteria for nitrification of the same wastewater. Hydrochloric acid was used to reduce the original pH of the wastewater which was 10.5 to about neutral range. The medium pH and alkalinity was raised by addition of 2 V/W % sodium carbonate to desirable level, which resulted in considerable ammonia oxidation. The effect of parameters such as feed concentration, inoculum size, initial pH, shaker's speed and temperature on nitrification was examined. It was observed while the medium temperature maintained at about 29 ˚C, shaker speed of 2.5 s-1, pH: 7.8 and inoculum size of 10 % V/V of 8th microbial consortium resulted in approximately 85% using the Watson medium and 80 % petrochemical wastewater nitrification. The data was fit to Monod type of equation and good agreement of 99.68 % was obtained.