Heat transfer is one of the important phenomena of human life and heat transfer devices are being developed every day. Low heat conductivity is the most common problem in conventional heat transfer fluids. This problem can be resolved by adding adequate amounts of nanoparticles. In this study, pressure drop and heat transfer on TiO 2 water based nanofluid flow was investigated experimentally. Three tubes with circular helical pattern with curvature ratio of 0.0375, 0.071, 0.123 and one square pattern with hydraulic curvature ratio of 0.071 was used with constant wall temperature. To employ constant temperature boundary condition, a steam chamber was used. Nanofluid volume concentrations of 0.1% and 0.5% were prepared using an ultrasonic device. Before testing heat transfer of nanofluids, the accuracy of experimental setup was assessed using water flow inside plane tube for both laminar and turbulent flow regimes. Results showed that for all working fluids and pipe types, with increasing Reynolds number, Nusselt number increases and the friction factor decreases. Using square pattern helical tube, the friction factor and Nusselt number significantly increased in comparison with circular helical pattern tube. For example, in laminar and turbulent flow in helically coiled tubes Nusselt number was increased to 9.6% and 4.9%, respectively. With the addition of nanoparticles to the base fluid, the results revealed an increase in friction factor and Nusselt number with increasing the volumetric concentration of nanoparticles. For square helically coiled tube, the volumetric concentration of 0.1% and 0.5% of nanofluid have caused increase of 10.3% and 16.5% compared with distilled water at the same Reynolds numbers, respectively. To determine thermophisical properties of nanofluids, some theoretical and correlated relations were used. Friction factor and Nusselt number show that, there is a rapid change in their values due to the transition from laminar to turbulent flow regime. The critical Reynolds number calculated here is much larger than plain tubes. Finally, using curve fitting approaches some empirical correlations for calculation of the friction factor and Nusselt number in hel ically tubes were introduced based on geometrical patterns, Reynolds number and Prandtl number. Keywords Pressure drop, convective heat transfer, nanofluid, helically coiled tube, constant wall temperature.