This numerical study aims to focus on the effect of difference between the inlet temperatures of working fluids on the hydrothermal characteristics of a counter-current minichannel hairpin heat exchanger. The water flows in the annulus side and the water based hybrid nano-fluid containing Fe3O4 and carbon nanotubes (CNTs) passes through the tube side of heat exchanger. Temperature-dependent thermal conductivity and viscosity are considered for the non-Newtonian hybrid nano-fluid. The effects of Fe3O4 and CNT volume fractions as well as the Reynolds number on the performance metrics of the heat exchanger are also assessed. The results revealed that the increase of difference between the inlet temperatures of working fluids leads to the augmentation of heat transfer rate, overall heat transfer coefficient (except at Reynolds number of 500), heat exchanger effectiveness and PEC; while the pumping power diminishes with the increase of inlet water temperature. (C) 2019 Elsevier B.V. All rights reserved.