Double immiscible flow has been extensively emerged in science and technology. In this research, mixed convection heat transfer of double immiscible fluids in a vertical parallel channel is investigated. One layer is pure fluid while the other is nanofluid. The effective kinematic viscosity and the effective thermal conductivity are regarded as linear functions of the nanoparticle loading parameter. A method of translation and scaling the computational domain for calculating the coupled equations subject to eight boundary conditions is developed. This method is applicable no matter the widths of the two regions are equal or not. Two interesting phenomena are revealed: the nanofluid with lower particle loading parameter is a better option for higher effectiveness of fluid driving; adding more nanoparticles in one layer can enhance heat transfer of both layers. (C) 2018 Elsevier Ltd. All rights reserved.