Slow sand filter is the most effective water treatment method and its performance depends on the raw water quality, pretreatment, design parameters and operational activities. Stayish town drinking water supply treatment sand filter has a problem in treating the water. The water becomes turbid and dark creates unhappiness in the users. The aim of this research work was to evaluate the performance of the treatment plant with seasonal variations. The study evaluated the treatment units design variables and selected physical, chemical and biological water quality parameters in summer (June 2020 to August 2020) and in winter (December 2020 to February 2021) periods. The samples were collected once a month for six months at the three selected sampling points; at the source, at the sedimentation and SSF outlets and water quality laboratory analyses were performed. Box plot and other plots, t-test, Pearson correlation, and mathematical calculations were used for data analysis. The raw water quality was absolutely different in summer from that of the winter. Essential design parameters and raw water quality were not within the sedimentation tank and SSF designing and operational guideline values. The sedimentation expected to remove 90% of contaminants especially physical pollutants before filtration but not did. It removes <30% of physical, biological and most chemical parameters. The SSF did physical and biological pollutants and also did chemical pollutant removal but vary with seasonal change. The treated water quality parameters values are nearly within the WHO drinking water quality guideline values except turbidity and too little difference of TSS in winter but not in summer. In general the SSF performance influenced due to high variation of the source water quality with seasonal change, pretreatment and design and operational problems. For well performance of the treatment plant source water pond should be constructed, operational activities should be done, water quality monitoring should performed regularly and for the future it’s better to reviewing the seasonal variations in water quality and other variables and applying pretreatment facilities. Key words:-Stayish drinking water treatment, sand filter, treatment units, drinking water quality, treatment performance, efficiency, water quality index