Gamma radiation is a physical process commonly used for the eradication of microorganisms distributed in food ingredients, medicinal plants and other bioresearches. The aim of this study was to investigate the effect of radiation dosage on the microbial load, chemical compounds and antioxidative characteristics of Polygoni Multiflori Radix (POMU). Ten commercial POMUs were purchased from different herbal markets and treated with 2 kGy, 4 kGy, 6 kGy, 8 kGy and 10 kGy gamma radiation doses to evaluate the microbial burdens of irradiated and unirradiated POMUs. Our results confirmed that 2 kGy was sufficient for the inactivation of enterobacteria; at 4 kGy, mold and yeast counts were obviously reduced; and at 6 kGy, neither yeasts nor fungi were observed any longer. The antioxidative effects and major antioxidant components of 0 kGy, 5 kGy, 10 kGy and 15 kGy irradiated POMU samples were also examined. Our results confirmed that 5 kGy irradiated POMU had both the highest antioxidative activity and lowest value in IC(50) of DPPH radical-scavenging activity. The content of total phenols had no statistically significant changes. Therefore gamma irradiation at 5 kGy could be a potential method for decontaminate the microbial load of POMU to prolong shelf life and to improve hygienic quality. (C) 2011 Elsevier Ltd. All rights reserved.