A metal cased well is chosen as the current electrode of resistivity tomography with a vertical line current source (RTVLCS), which greatly deepens the depth detection of resistivity tomography and improves the resolution of resistivity tomogram. RTVLCS can be carried out during the course of oil production, and using the apparent resistivity data acquired by this technique and inverse computation, the true resistivity of a target layer can be obtained, then the distribution of residual oil saturation via the relationship of resistivity and oil (or water) saturation. Analyzed our result with these from geological analysis, tracer detection method and rock core analysis and so on, it could provide an important reference for drawing up a plan of oil production. At last the goals of stable and increasing oil production can be achieved. In the present paper, RTVLCS has been tested in a mid-or post-phase oil region of an oilfield, and the distribution of residual oil saturation in its target stratum has been obtained. Compared this result with the distribution of the observed water saturation of each well in the oil region during the course of the measurement of resistivity tomography, it is found that the residual oil saturation obtained by this technique is reliable, which can be taken as the important reference of drawing up a plan for oil production in the mid-or post-phase of an oilfield. Therefore, RTVLCS has great application potentiality.