The availability of land and the climatic conditions encouraged by the geographical situation of Morocco, have pushed this country to adopt since the 90s natural leaning as being the technical solution most suited to the economic and climatic context, thus, many treatment stations wastewater has already been carried out. In order to assess the performance of the natural lagoon-type treatment plant in the town of Outat El Haj, we studied the physicochemical parameters of the raw and purified water from the plant. For this, we carried out a set of measurements such as: Temperature, pH and Electrical Conductivity (EC) (parameters on site), Biochemical Oxygen Demand (BOD<sub>5</sub>), Chemical Oxygen Demand (COD), Suspended Solids (TSS), Dissolved Oxygen, Hydrotimetric Titer (TH), Cl- Chloride, Organic Matter (OM), Orthophosphates, and Nitrates, in order to examine the reliability of this purification process in the study area. The purification yields observed in the treatment plant vary from 23 to 94.6% as an abatement for BOD<sub>5</sub>. This average yield varies between 22.3 and 89.8% for COD, between 22.6 and 90.6% for OM, and between 9.2 and 65.5% for suspended matter (suspended matter). The purification performance is negative to very low in nitrogen and Orthophosphates. The results of the analyzes showed that the purification yields of the natural lagoon-type station, low, variable and remain dependent on the season.