Abstract: Under simplified conditions, an estimate of the probable order of magnitude of the piezo-magnetic effect has been made. It follows that magnetic field variations observed before asd after an earthquake is unlikely caused by piezo-magnetic effect in not magnetically anomalous areas. Piezo-magnetic effect is not the only factor giving rise to the variation of the geomagnetic field in connection with the occurrence of earthquakes.The formulas derived from the theory of diffusion current show that the so-called dilatancy-magnetic effect caused by ground water diffusion is also very small. If the area of ground water diffusion is sufficiently large and the movement of water is very rapid, the dilatancy-magnetic effect may be detected.