Due to the ever-increasing importance of energy, research in the field of heat transfer is always one of the up-to-date issues. In this study, heat transfer enhancement and excessive pressure drop are determined numerically in a helical pipe with flattened cross-section filled with a nanofluid. To accomplish this, several cases are considered with six different aspect ratios of the cross-section geometry with different orientations, as well as four different Reynolds numbers in the laminar flow regime. Alumina/water 1% is considered as the working nanofluid and the 'four equations' two-phase model is applied to render the velocity, temperature and concentration distributions. The results indicate that; the flow in the horizontal cross-sections, as well as the flow in the circular cross-section, has only one pair of vortices. However, the flow in the vertical cross-section induces more than one pair of vortices. The thermal performance of the helical coils with flattened cross-section is higher than that with the circular cross-section for the same working fluid. In addition, the thermal performance of the same helical coil filled with nanofluid is greater than that with water. However, modifying the geometry is more effective on the heat transfer enhancement in comparison to changing the working fluid. When Re = 360, the heat transfer enhancement due to the use of the nano-fluid is 1.4% compared to water and it is augmented up to 6% by modifying cross-section from circular to flattened shape. It is also shown that the heat transfer augmentation due to the cross-section modification becomes more effective in lower Re numbers. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.