Total length of workers around 2 - 3 mm. Head subtriangular, with rounded posterior corners, concave posterior margin and flattened dorsum.
Mandibles elongate, subtriangular, with numerous small teeth.
Anterior margin of clypeus more or less concave. Spongiform appendages present on petiole and postpetiole. No erect hairs on head, mesosoma or petiole. In Japan the genus is separable from Smithistruma by its flattened head and mandibles and hairless mesosoma. Both species of Pentastruma occur in Japan, but they are rare.