Twisted-tape is one of the most common method of passive heat transfer augmentation techniques for forced convection. In this paper, a numerical investigation of the heat transfer and pressure drop under natural circulation for in-tube twisted-tape geometry is performed using computational fluid dynamics (CFD). Three-dimensional CFD models were simulated for a plain tube (9 mm inner diameter with 100 mm length) and compared with identical tube fitted with full-length twisted tape, regularly spaced twisted tapes (with spacing of 2 and 3 times the tube diameter), and different widths of the twisted tapes (9 mm, 8 mm and 7 mm) inserted with several twist ratios (pitch over diameter (p/d) = 2.7, 3.3, 6, 7.2 and 25). The results revealed a strong effect of twist ratios on the inner wall temperature. The results have also shown that the heat transfer coefficient and pressure drop increase with the decreasing twist ratio. For full length twisted tapes with p/d = 2.7, a 28% and 102.8% increase in heat transfer and pressure drop were observed, respectively. For the case of regularly spaced twisted tapes, an increase in heat transfer and a pressure drop with a decreasing space ratio was observed, including an increase in the heat transfer coefficient of 27.4% and 26% and a pressure drop of about 96.2% and 90% for inter-tape spacing equal to two times the diameter and three times the diameter, respectively for p/d = 2.7 as compared to the smooth tube without twisted tapes. For the case of different widths of the twisted tapes, it was found that the heat transfer coefficient and pressure drop increase with increasing widths. The heat transfer coefficient was enhanced by 25.8%, 27% and 28%, and the pressure drop increased by 93.8%, 97.4% and 102.8% for p/d = 2.7 with 7 mm, 8 mm and 9 mm widths, respectively.