A CFD and UV fluence simulation program has been developed to evaluate the performance of drinking water disinfection reactors. Based on the flow field and the UV intensity field, the UV fluence or dose is obtained by solving the convection-diffusion partial differential equation of the fluence in the reactors. Eight UV-disinfection reactors from four manufacturers were evaluated. The fluence at the reactor outlet was evaluated using a logarithm microbial survival rate function. A calculated reduction equivalence fluence CEF was defined at the outlet and compared with the average fluence. That leads to a definition of a disinfection efficiency of the reactor. It is found that while the accurate prediction of the fluence level is difficult due to the difficulty in obtaining accurate lamp efficiency, quartz transmittance and reactor wall reflection rate etc., the accurate prediction of disinfection efficiency is quite possible because the uncertainties mentioned above have much less influence on efficiency prediction. The reactor's disinfection efficiency is mainly determined by the flow pattern, the lamp number and the lamp locations. These are primarily mechanical design issues for design optimization. Therefore the CFD and fluence modelling method presented in this paper can be useful for reactor design optimization.