Abstract: In monitoring water- or mud-injection of oil well by electrical method, the potential electrodes are placed on the surface and the corresponding current electrode is the steel case of oil well which is a vertical line source of current. The purpose of exploration is to reveal the flow direction and distribution of fluid through these observed data. 3-D resistivity inversion of a vertical line source of current and a method of constructing Jaccobian matrix are introduced in this paper. A preliminary result from a test oilfield is presented. An oil production well is taken as a vertical line source of current, around which 108 electrodes are placed evenly at 6 circles with 50 m space in radius. The medium true resistivity values of 5 horizontal layers ranging from 500 m to 2 000 m deep are obtained. The result shows that this method is effective to monitor the direction and distribution of fluid in a certain layer.