A three dimensional numerical simulation has been carried out to study the effect of porous media on the heat transfer and pressure drop of magnetite (Fe3O4/water) nanofluid flow in two conventional heat sinks; circular and rectangular channel heat sinks. Effects of different parameters such as nanofluid volume fraction and flow rate and channel porosity on the performance of the heat sinks have been investigated. A maximum heat transfer enhancement of 13% and 14% has been achieved by using nanofluid in clear rectangular and circular channel heat sinks, respectively. These values grow up to 450% and 547%, respectively when the heat sinks are filled with the porous media. The heat transfer enhancement due to the insertion of the porous media is accompanied by very large pressure drop. Results indicate that the overall heat sink performance is improved at low flow rates and porosities and high concentrations of the nanofluid. (C) 2018 Elsevier B.V. All rights reserved.