Abstract: The aim of the present study was to study the effect of long-term exposure of rats (1 hour per day, 3 days weekly for 4 weeks) to either static or time varying magnetic fields of induced flux densities 2 mT (millitesla) =20G (Gauss) , on the ultrastructure of liver tissue. One hundred and ten male rats were divided into three main groups. Animals of the first group (10 rats) were not exposed to the magnetic field and represented the control group. The second group (50 rats) was exposed to static magnetic field (direct current) at strength of 2 mT. The third group (50 rats) was exposed to alternating magnetic field (alternating current) at strength 2 mT. The results revealed structural irregularity in hepatocyte nuclei as the most prominent ultrastructural change in the liver of treated groups. This was manifested as irregularity of nuclear membranes, widening of the nuclear pores and heterogeneous distribution of the chromatin material. Furthermore, swelling and clumping and deformation of mitochondria were observed in the groups exposed to the magnetic field. In addition, the rough endoplasmic reticulum appeared with marked dilation and the lysosomes appeared distorted.