This invention discloses a reagent composition for a biosensor having high sensitivity which is capable of improving analysis linearity of an analyte such as glucose by reacting an oxidoreductase, a metal-containing complex, and Naphthol Green B, and minimizing blood necessary for measuring blood glucose since it is possible to detect a concentration of a small amount of the analyte, and a biosensor having the same. The reagent composition for a biosensor includes at least two kinds of electron transfer mediators including Naphthol Green B, and an enzyme.