To study the ultrastructural pathological characteristics of vascular and connective tissues in primary syphilis. The histomorphological changes in the dermis were examined with transmission electron microscopy. Capillaries with a thickened multilayered basement membrane,occluded lumen, and proliferation of endothelial cells were found. The spirochetes entered the capillary vessel through the basal membrane and endothelial cells, and were separated or attached to red blood cells.Large numbers of inflammatory cells were found around the blood vessels.There were T.pallidum in the cytoplasm.The pathological changes in the capillaryies and connective tissue were morphological basis for the study of pathogenetic mechanism of primary syphilis.