This paper investigates the three dimensional flow field and organism concentration in a UV disinfection channel. The modified P-1 model is newly adopted to solve the UV intensity field. This modified P-1 model for radiative transfer can yield an accurate light intensity for even complicated geometries. The effect of UV light intensity on the disinfection of microorganisms has been considered in the species equation. Differently from existing literatures, this study has used the Euler method for the calculation of both the flow field and the concentration of organisms. The CFD modeling results are compared with existing experimental data [1] for a UV channel, which shows outstanding agreement. The effects of inlet velocity, absorption coefficients of lamps quartz and water on the disinfection efficiency are investigated. With a smaller inlet velocity and smaller absorption coefficients, higher disinfection efficiency has been obtained. The current CFD model has the capability to predict the performance of UV disinfection channels.