Argulus japonicus Thiele, 1900 was originally described from China, but has spread to Europe, Africa, Australia, and North America. An ultrastructural description of A. japanicus has been done as one of the most common fish ectoparasites which has a high affect on fish life. The description was based on light and scanning electron microscopes studies using 13 females and 7 males which were collected from Shunde city in Guangdong Province, China. Typical Argulus body and body's appendages are associated with our specimens, carapace covers fourth leg, and scales separate uniformly on frontal and lateral lobes, ventral surface of thorax is ornamented by scales contrary of the dorsal surface which is quite smooth, various shapes of scales have been detected on second maxillae. Location and structure of copulatory accessory, number and structure of overlapping sclerites of sucking disk rim, melanophores patches of male, second swimming leg structure, and parasite's total size are the most important criteria which may be used to distinguish between male and female. Moreover, new classification criteria of Argulide, obtained by scanning electron microscopic study of previous species, are discussed.