In this study, we showed that smaller size particles of Nano-Se have better scavenging effects on the following free radicals: carbon-centered free radicals (R * ) generated from 2,2'-azo-bis-(2-amidinopropane) hydrochloride (AAPH), the relatively stable free radical 1,1-diphenyl-2-picryhydrazyl (DPPH), the superoxide anion (O 2 * - ) generated from the xanthine/xanthine oxidase (X/XO) system, singlet oxygen ( 1 O 2 ) generated by irradiated hemoporphyrin. Furthermore, the three sizes of Nano-Se studied also show protective effects against the oxidation of DNA. The three samples all have potential size-dependent characteristics on scavenging the free radicals. Although in this study we regarded Nano-Se as a whole without considering interactions between BSA and the red selenium nano-particles, there is the possibility that the apparent free radical scavenging effects may be partially contributed by such interactions.