A fixed bed reactor that operates in gas-liquid co-current down flow is called Trickle Bed Reactor (TBR). In this study, authors employed the lab-scale TBR, made of 68, 100mm inner diameter acrylic column, packed with particles of two sizes (3, 5 mm) that are used in the actual reactor. Liquid and gas were injected from the top of the column and cross-sectional liquid distribution was captured at the bottom of the column by electrical resistance tomography (ERT). ERT is a tomographic technique that provides the cross-sectional conductivity distribution at the rate of about 50 frames per second by injecting current and measuring voltages between the 16 electrodes that are attached around the column. By analyzing the spatial and temporal characteristics of the liquid distribution obtained by ERT, the relationship between operation condition and flow regime transition and property of pulsing flow was revealed.