AbstractWe used the method of resistivity tomography to earthquake monitoring. Through inversion of apparent resistivity, the image of true resistivity was reconstructed. It reveals the nature of variations of the relevant physical parameters during seismogenic process. At two stations in North China, Baodi and Changli, obvious anomalous changes appeared in the tomograms 6 days before two earthquakes of ML5.0 and ML4.4, respectively, which occurred in Tangshan on 14 April 1998. The distribution of the true resistivity in the images of the Baodi station showed a new pattern, i.e. an ordered state, rather than an ordinarily stable pattern with an even background. The resistivity image of the Changli station showed distinct high‐ and low‐resistivity areas and an ordered pattern, too. After these earthquakes, the above mentioned phenomena all disappeared, while the basic image pattern related to the geological structure beneath the stations did not change much. As an indicator of the ordered state of an image, the entropy of tomograms at two stations all decreased distinctly before the earthquakes, which might be the precursor to these earthquakes. It is inferred that these resistivity anomalies were mainly attributed to the changes of the electrical‐conducting structure and porosity in the soil layer under increasing tectonic stress before earthquakes. The present result shows that in a conventional apparent resistivity survey with one fixed electrode spacing the observational results are not sensitive to the change of true resistivity and have no basic spatial resolution. These shortcomings result from the essential property of apparent resistivity which is an integral value and a potential function. Introducing resistivity tomography into the earthquake monitoring and improving present observational systems are necessary to develop the methods of earthquake prediction.