The biological activity of activated sludge in the presence of synthetic wastewater containing pollutant of phenol was investigated. The wastewater was a mixture of molasses, phenol, urea, and ammonium phosphate. Two types of synthetic wastewater were prepared with a total COD of 1000 mg/l, the first one contained molasses, and the second one was a mixture of molasses and phenol. For the sludge fed with the mixture of molasses and phenol, the phenolic COD was gradually increased from 100 to 1000 mg/l within 10 days, and the COD changes, phenol concentration, and Colony Forming Units (CFUs) per unit volume in 24-hour intervals were monitored. The results showed that the changes in the COD and the CFUs follow the same pattern in the presence and absence of phenol, and phenol removal is complete for the phenol-contained wastewater. The impact of shock loads of phenol on the phenol removal performance was examined by feeding sludge by the wastewater of phenolic COD of 500, 700, 800 and 1000 mg/l. It was observed that for the wastewater containing only phenol (and not molasses), increasing the phenol concentration from 216 to 423 mg/l decreased the COD degradation from 90% to 5% in 24 hours. Meanwhile, phenol degradation in the aforementioned intervals decreased from 100% to 5%. At the same time, the response to shock loads of high concentration of phenol in presence of other sources of carbon besides phenol was investigated by feeding other sludge by the wastewater of the a COD of 1000, 1400, 1600 and 2000 mg/l due to phenol and molasses with the same COD proportion. The results indicated that for the wastewater containing both phenol and molasses, increasing the phenol concentration from 216 to 430 mg/l decreased the COD degradation from 92% to 62% in 24 hours