The performance and hydrothermal characteristics of a biologically produced nanofluid in a miniature counter-flow double-tube heat exchanger are investigated considering particle migration. Hot water flows in the annulus side, and the nanofluid is employed as the coolant in the tube side. The particles used are silver nanoparticles synthesized through plant extract method from green tea leaves. Particle migration disturbs particle concentration distribution, and a more non-uniform concentration is developed by increasing either Reynolds number or mean concentration. The results reveal that at great concentrations and Reynolds numbers, particle migration possesses a significant effect on efficacy of the heat exchanger. Particle migration increases the heat transfer rate while decreasing pumping power. As concentration and Reynolds number increase, the overall heat transfer coefficient and heat transfer rate enhance. The effectiveness and number of transfer units decrease by increasing Reynolds number, and augment with concentration increment. Pumping power significantly intensifies by increasing Reynolds number, but it reduces by augmenting concentration, which is a very positive finding. In addition, the ratio of heat transfer rate to pressure drop increases with concentration increment and therefore, the nanofluid has greater merit to be applied in the heat exchanger at higher concentrations. Moreover, the overall heat transfer coefficient enhances by increasing the water temperature in annulus side inlet, which is due to an increase in convective heat transfer coefficient of the tube side because of the nanofluid thermal conductivity augmentation. (C) 2017 Elsevier Ltd. All rights reserved.