Background: The phrase "No life without water" emphasizes the fundamental role that water plays in sustaining life. Water is a dynamic system, consisting of living and non-living components, and it can contain various organic, inorganic, soluble, and insoluble substances. Some of protozoa found in water are Entamoeba histolytica, Giardia lamblia, Cryptosporidium paravum, Paramecium caudatum, Amoeba proteus, Balantidium coli. Objectives: Seasonal Effect of Physico-chemical Parameters on Protozoa in Drinking Water of Kathmandu Valley, Nepal. Materials and methods: Water samples were normally examined within 2-6 hours of collection using the centrifugation method ( 2,500 r/min for 1-2 minutes) followed by light microscope to detect cysts and oocysts of the different parasites. Direct smears were examined under the light microscope at 100x magnification. Lugol's iodine stain and Modified Ziehl-Neelson Staining were applied to conform the presence of protozoan cysts and oocysts. Results: The analysis of protozoa presence in water samples across different seasons reveals significant variation in their numbers and percentages throughout the year with F value 18.571 (P-value<0.001). The significant highest number of protozoa 17 (2.36%) were found in the summer season. In spring, 6 protozoa were detected, making up 0.83% of the total (P-value<0.001). In autumn, 10 protozoa were found, representing 1.38% (P-value=0.03), and in winter, 9 were detected, constituting 1.25% (P-value=0.01).These findings suggest that protozoa population fluctuate seasonally, likely influenced by environmental factors such as temperature, nutrient availability, and water conditions, which vary over the year. The analysis reveals significant seasonal variation in protozoa populations, with the highest number occurring in summer, and statistically significant difference observed across all seasons. The physico-chemical parameters were analyzed by APHA method 1992 Edison 18. Conclusion: The water treatment is necessary to eliminate microbial, chemical contaminants which reduce the risk of water-borne diseases.There is a need for better management of the surrounding environment of water reservoirs and feeding streams to prevent contamination. This includes implementing protective measures to reduce the introduction of pollutants into groundwater sources.