Abstract: By measuring the change in current with time in the primary coil of a transformer-type sensor, the authors investigated the change in the electrical resistivity of hypo-eutectic white cast iron, grey cast iron and rare-earth magnesium cast iron during solidification. It has been found that the features of these changes of various cast irons are quite different, depending on the behavior of solidification and the crystal structure of the phase crystallizing in the course of solidification. The crystallization of austenite and spheroidal graphite will cause a decrease in the resistance of specimen, or a current increase in the primary coil, while ledeburite or flaky graphite brings about a change in opposite direction. Among all phases present in cast iron, the graphite is a predominant factor determining the change in resistivity. It is convinced that the results reported in this paper are helpful to the study of solidification mechanism and production control of cast iron.