Abstract: The properties of rock resistivity were studied under pressure, particularly with stress reversal procedure, in which the applied pressure was increased, decreased, and increased again.It was observed that, 1) with pressure increasing, the main feature of the resistivity changes was increase-steady-decrease for high saturation rock samples (saturation 71-100%). But the main feature for low saturation samples was different (resistivity increased). 2) In 10 out of 11 cases for high saturation samples the resistivities droped remarkably (about 2%). These drops could explain the negative anomalies commonly observed in geoelectricity surveys, before earthquakes in China. 3) It was also observed shortly before rock failure that, a) The resistivity drops much more dramatically in the stress reversal period, about 20% more than ordinary drops, b) These drops occured not only during stress decrease but also during stress increase, c) The anisotropic property of resistivity change is enhanced. 4) Resittivity increase was observed during the stress reversal period for low saturation rock samples.The results mentioned above were explained with the effect of water flowing in and out of the cracks of rocks. Temporary factors, which bring about a reduction of the main stress, may enhance the possibility of earthquake occurrence.