A new concept on the enhancement of forced-convection heat transfer at a Reynolds number lower than 3000 is presented. A new surface has many holes and is bent in a trapezoid shape. The phase of each adjacent surface is changed by half a period, and therefore a duct constructed with these surfaces has enlargement and contraction parts alternately along the flow passages. The heat transfer coefficient of the new duct increased by about three times that of a duct with plain surfaces at the same Reynolds number.