Small molecules of free radicals play essential roles in maintaining normal physiological function in organisms. However, in a variety of pathological conditions, excessive accumulation of these substances might cause serious damage to tissues and organs due to their highly active, strong oxidization properties. For instance, reactive oxygen species (ROS) and reactive nitrogen species (RNS), two major small molecules active substances, are proved to participate in the pathogenesis of stroke. In particular, the role of reactive nitrogen in stroke onset is one of the hot spot in current stroke etiology study. In this publication, recent progress of the physical and pathological functions of RNS in stroke study are reviewed and prospected.