An experimental investigation on compound heat transfer enhancement by a combination of a helically ribbed tube with twisted tape inserts was performed. The measurement of heat transfer coefficient as well as the pressure drop was performed in order to evaluate the performance of enhanced tubes using the equal pumping power evaluation method. The heat transfer coefficient was obtained by means of Wilson's method for the Reynolds number ranging from 300 to 30,000. The combination of smoothed tube and twisted tape having a clearance NDC = 0.07 accomplished the highest performance in the laminar region. On the other hand, the combination of helically ribbed tube and twisted tape having a clearance NDC = 0.02 gave the highest performance in the turbulent region. The reasons for these results are discussed well in terms of the compound effect and clearance effect.