Abstract: Histology and ultrastructure of the kidney in Swordtails（Xiphophorus helleri） were studied with microscope and transmission electron microscope. Resembling other teleost, the mesonephros was consisted of renal corpuscle and renal tubules. Renal corpuscles were interspersed in the kidney. The renal tubules were composed of the neck tubule, first and second proximal tubule, distal tubule and the collecting tubule. Lymphoid tissues, including Lymphocyte, Monocyte and Melano-macrophage, were differentiated in the kidney under electron microscope. As a multifunction organ, the characteristic of structure and ultrastructure of the kidney in fish were also discussed in this paper.