Polygonum L. s. str., belonging to Polygonaceae family, is a big genus with abundant medicinal plants.  More than 10 plants are specified in Chinese Pharmacopoeia and many local medicinal standards and over 50 species are used as folk medicines.  Owing to the similar morphologies and very small flowers and fruits, they are uneasily identified and often confused with each other and misused clinically.  In order to provide a basis for identification of Polygonum s. str. plants, a histological study on stems and leaves of 30   species from Polygonum was undertaken by a routine/polarized light microscopy for the first time.  The results showed that: ① the transverse sections of stems of Polygonum are relatively similar, sclerenchyma such as xylem and fibres with strong polarization effects; ② the surface views of leaves of Polygonum are distinguishable on distributions and types of stomata, with or without attachments (such as glandular hairs/scales or non-glandular hairs) and the polariscopic features of epidermal cell walls, stomata and cell contents.  Observed under polarized light, it was found for the first time that stomata on leaf surface of some plants have a Maltese-cross effect with the arms of the cross intersecting at the stomatal opening.  As a result, a key combining the microscopic and polariscopic characteristics of the stems as well as leaves was provided for identifying the 30 medicinal plants of Polygonum.  The polarized light microscopic method was proven to be one of the quick, simple and effective techniques for the identification of medicinal plants and botanic crude materials.