The flow fields inside the photo-catalytic module for water treatment system were studied. The fluid flow inside the module was visualized mainly by CFD(computer fluid dynamics) analysis, and theh numerical analysis was authorized by PIV(particle image velocimetry) measurement. The flow patterns in the module for 3 types of innards and 6 kinds of flow rate were analyzed by CFD. Model 1 had better performance below 0.1 L/s of the flow rate, and model 2 over 0.2 L/s of the flow rate with the homogeneous flow field innards of photo-catalytic module.