Abstract: The electric resistance behavior of pitch-base preoxidized fibers during the carbonization is measured. The conclusions have shown that the fiber's resistivities during the carbonization decrease from 10¹² Ω-cm to 10^-1 Ω-cm. And the quantitative relation between the fiber's resistivities and the carbonized temperatures is got. Using the relation of fiber's resistivities and the results of fiber's element analysis infers the constructive changes of the fiber during the carbonization. The example of the carbon fiber's electric conductivity used has been given.