To study enhancement in heat transfer by corrugated tubes, one smooth tube and seven kinds of three-starts spirally corrugated tubes which had different combinations of the depth and pitch of corrugation were used in a double-pipe heat exchanger. The friction factors and heat transfer coefficients of the tube inside and annulus were measured, and the contribution of the corrugated tubes to the enhancement in heat transfer was examine The spirally corrugated tubes provided a secondary spiral fluid motion near the tube wall. The effects of the geometrical parameters were expressed by the tube severity factor which is a function of the depth and pitch of corrugation and the tube diameter. With increasing severity factor, both the friction factors and heat transfer coefficients of the tube inside and annulus increased. The contribution of the corrugated tubes was examined by evaluating the overall performance which was defined as the ratio of the enhancement in heat transfer to the increase in friction factor. A maximum in overall performance appeared at a relatively low severity factor.