Experimental investigations of friction factor and heat transfer characteristics through a square duct fitted with helical screw tapes of various twist ratios have been studied under nearly uniform wall temperature conditions. The experimental results reveal that the use of helical screw tapes leads to a considerable increase in heat transfer and friction loss over those of a plain square duct. The Nusselt number increases with the rise of Reynolds number and the reduction of twist ratio. The Performance evaluation criteria have been presented and maximum performance ratio was observed for minimum twist ratio.