This study investigates the hydrodynamic and thermal performance of nanofluids in the region of thermal developing laminar mixed convection. Single-phase approach was employed to evaluate the pumping power and heat transfer coefficient, i.e., no novel mechanism other than the renewing thermophysical properties of nanofluids was involved. An aggregation-mechanism-based methodology was introduced and verified to well unify the theological and thermal properties of nanofluids. In comparing the pumping power savings, critical differences between forced convection and mixed convection were found: nanofluids are more favorable for application to mixed convection. Criteria for nanofluid forced-mixed transition have been discussed and presented to help the decision of whether a flow can be viewed as mixed flow. (C) 2013 Elsevier Ltd. All rights reserved.