A GIS based hydrologic model, SWAT (Soil and Water Assessment Tool) was applied for modeling the WeruWeru catchment at the foot slopes of Mt. Kilimanjaro in Northern Tanzania. The catchment has an approximate drainage area of 101 km2 and a mean annual precipitation between 1500mm and 3000mm. The water balance modeling was performed on annual and monthly bases using spatial and temporal data. A statistical weather generator file WXGEN was prepared for ten years to generate climatic data and fill in gaps in the measured records of climatic data. Various GIS data preprocessor modules involving watershed delineation, input map characterization and processing, stream and outlet definition, the computation of the geomorphic parameters, and characterization of the landuse/land cover and soil were developed in the course of modeling. Surface runoff computation was done using Soil Conservation Service-Curve Number (SCS-CN) method; and Muskingum routing method was used for flow routing. The Rainfall-Runoff modeling was based on a long term global water balance simulation for 15 years (1972-1986) and temporal calibration technique. The Nash and Sutcliff efficiency criterion (R2 ) and the Index of Volumetric Fit (IVF) were adopted for the measure of efficiency of the performance of the model. An R2 of 82% and 59% was obtained during calibration and verification periods respectively. The predicted mean daily stream flow was found to be 1.92m3 /s exactly as observed during the water balance simulation. Besides, modeling result gave a total annual water yield of 597.1mm, from which the annual surface water component was 155.8mm and that of the base flow component was 441.4mm in the long term simulation period with IVF unity. While demonstrating the catchment is rich in ground water sources as a result of high magnitude of precipitation and good water retention capacity, this study shows that SWAT model can be a potential monitoring tool for watersheds in mountainous catchments.