Turbulent heat transfer and friction factors are measured in this paper for 16 internally grooved tubes with different geometrical parameters. Experiments are conducted for the 16 tubes with the Reynolds number range from 10,000 to 100,000 and Prandtl number from 4.98 to 8.22. Other parameter ranges are: 1 <= N-s <= 45, 0.016 <= e/d(i) <= 0.04, 13 <= alpha <= 45, where N-s is the number of circumferential micro-fins (number of starts), e and alpha are the height and helix angle of the micro-fin, respectively, and d(i) is the inner diameter of the embryo tube. An equation for predicting the average heat transfer of the inner helically ribbed tubes is presented based on Gnielinski equation with the friction factor in the numerator of the original Gnielinski equation being replaced by the measured friction factor in the fully developed flow region of the internally grooved tubes. For all data of the 16 tubes, most of the relative deviation is within +/- 10%. Comparison of this equation with other data available in the literature is also provided, and the deviation of more than 93% of the compared data is within +/- 20%, 99% within +/- 40%. Since the friction factor is easier to be measured, the proposed correlation equation is practically very applicable and its accuracy is also acceptable for the engineering design. (c) 2011 Elsevier Ltd. All rights reserved.