Faecal coliform and faecal Streptococci die-off was investigated in the surface flow granular bed constructed wetland cell. The granular bed cell of effective plan area of 7.5 m × 1.5 m and 0.7 m deep was filled with aggregate size 19 mm up to a depth of 0.35 m. The remaining 0.35 m depth was filled with water. To assess the performance of the granular bed, samples were collected at the influent and effluent of the cell for calibration of a mathematical model. The analysis of the model suggests that bacterial mortality rate constant was largely influenced by sunlight intensity, which was responsible for 72.6% of removal. The influence of other parameters such as interception, filtration, dissolved oxygen, temperature, sedimentation on bacterial mortality rate were less significant. For instance, dissolved oxygen and pH contributed 7% and sedimentation only 0.44%. The ratio for the rate constant term to light attenuation coefficient (ks/K) was 1.281 × 10-3 m cm2/cal and the model efficiency was 0.80.