The Sewage Treatment Plant (STEP) based on Skhirat City receive on average an important volume of wastewater (about 66473 m3 with a flow of 2173.56 m3/day). The aim of this research is to evaluate the pollutant load of wastewater and the performances of the sewage treatment plant of Skhirat city in treating domestic wastewater. Sampling was carried out on the level of the entry of sewage treatment plant with a monthly rate. The analyses were carried at the laboratory with the cubic measure of wastewater received by the sewage treatment plant. The results show that the CODe/BOD5e ratio of 2.08, which makes it possible to confirm the normal state of the station in biological treatment since the ratio is lower than three. The results showed that the rate of the Chemical Oxygen Demand (COD), Biochemical Oxygen Demand (BOD5) and the Suspended Matter (SM) respectively it’s of the order 87,4%, 90,68% and 79,50%. Globally, the results of the investigation revealed by the unilateral test Student, that this difference between the entry and the exit of the station which significant by p-value calculated is lower than the level of significance alpha= 0,05.