The most up-to-date account of Diplazium in Thailand, including 29 species, is in Flora of Thailand, vol. III. Recently, some new recorded species were reported. However, the key to the species in the Flora of Thailand did not include these new records. Moreover, it is difficult to determine the species using this key, due mainly to the ambiguity of the key characters. The account of the Thai Diplazium, therefore, is needed to be re-examined. This research was designed to explore additional data for species determination and key construction by examining the morphological, anatomical and palynological characters of the available species. In this study, 7 morphological and 2 anatomical characters of the Thai Diplazium were examined. Both fresh plants and herbarium specimens were used. It was found that growth form of rhizome, indumenta, stipe and lamina characters together with anatomy of stipes are collectively useful in Diplazium taxonomy with special reference to scales morphology and shape of vascular bundle of upper portion of stipes. A palynological study of 31 species was undertaken using light and scanning electron microscopy. All studied species have monolete spores, which are bilateral, kidney-shaped, 30-68 by 20-50 µm and bearing perispore. The spores sculpture can be divided into 8 types: baculate, echinate, pustulate, reticulate, rugate, rugate-retate, rugulate and smooth types. These microscopic characters can be used to identify different species. Thirty one species of Thai Diplazium are enumerated and discussed. Three of them, namely D. griffithii T. Moore, D. lobatum (Tagawa) Tagawa and D. pallidum (Blume) T. Moore are new records for the Thai Flora. Diplazium simplicivenium Holttum was treated here as a synonym of D. dilatatum Blume, while D. petilotii Tardieu is excluded from this account. Key to the species together with descriptions, line drawings and distribution of each species are provided.