The present study is devoted to the mixing time investigation in a single stirrer UV photoreactor aiming at the drinking water disinfection process. Electrical resistance tomography (ERT) was employed to measure the mixing quality due to the significant advantages. The reactor was a flat-bottomed cylindrical tank with a diameter of 38.1 cm and a height of 60.1 cm fitted with four symmetrically located vertical baffles. The performaces of a 6-blade Rushton turbine and a 4-blade 45° pitched-blade turbine were explored in this study. In the absence of the UV light, four PVC rods were used to replace four UV lamps and evaluate the impact of the locations of the UV tubes on the mixing time. The experimental results demonstrated the feasibility of the ERT system to monitor the mixing process in the UV photoreactor. The ERT results also indicated that the locations of the UV tubes had a signigicant impact on the mixing performance in such a batch stirred tank reactor. Other parameters encompassing the impeller rotational speed the impeller type, and off-bottom clearance were presented with respect to the extensive effects on the mixing time and power consumption.