Field-synergy analysis is performed on the viscous dissipative water-alumina nanofluid flow in circular microchannel heat sinks to scrutinize the synergetic relation between the flow and temperature fields for both heating and cooling processes. By varying the Reynolds number and the nanoparticle volume fraction, the effect of viscous dissipation in the convective heat transfer of nanofluid is investigated based on the field synergy principles. For the heating process, under the effect of viscous dissipation, the degree of synergy between the velocity and temperature fields of nanofluid flow deteriorates, leading to a dwindled heat transfer performance of the nanofluid. Due to the presence of two difference heat sources, the synergy angle and field synergy number for viscous dissipative flow are defined to characterize the synergetic behavior. The field synergy in the nanofluid is greater at low-Reynolds-number flow due to the reduced effect of viscous dissipation. By reducing the size of the nanoparticle and increasing the diameter of the microchannel, the degree of synergy between the velocity and temperature fields of the nanofluid flow in microchannel can be intensified, yielding convection heat transfer enhancement. On the other hand, for nanofluid cooling process, the presence of viscous dissipation would augment the field synergy of the flow. (C) 2014 Elsevier Ltd. All rights reserved.