The work is to observe effects of PEMW (pulse electromagnetic wave) exposure on function and morphology of thyroid gland of rats. At different time points (24, 48, 96 and 192h) after exposure to PEMW (E=115kV m,-1, 12000 pulses), radioimmunoassay was performed to observe the levels of thyroid-stimulating hormone (TSH), thyroxine (,T,4,) and triiodo- thyronine (,T,3,) in sera of the male Sparague-Dawley rats. Optic microscopy and transmission electron microscopy (TEM) were used to observe structural changes of the thyroid gland at the time points. The ,T,3,、,T,4, and TSH in sera increased significantly, reaching a peak at 24h and decreasing gradually then. Although no obvious changes in the thyroid gland were observed under the optic microscope, some ultrastructral changes in the thyroid gland were found under the TEM. The ultrastructures were obviously changed at 12h and aggravated until 48h. In the experimented rats, dilatated endoplasmic reticulum gathered with lots of protein excretion, lipid droplet and heterochromatin gathered under the nucleus membranes were observed in follicular epithelial cells. These changes were palliated at 96h but not recovered. PEMW can affect levels of hormones in sera and the hazard ultrastructual changes of thyroid gland. Endoplasmic reticulum is the main injured organelle.