The primary working mechanism behind nanofluids thermal conductivity can be distinguished as one of the most controversial issues of nanofluids. Between several theories proposed in literature, nanopartides' clustering is the sole mechanism that can be observed even at the micro-scale level. Several parameters including nanofluid preparation method, particles concentration, particles morphology, temperature, and elapsed time are experimentally altered to investigate the role of nanoparticles micro-clusters in nanofluids thermal conductivity. It is observed that a minor variation in the altering parameters, would lead a remarkable change in the micro-clusters configuration and subsequently, in the thermal conductivity of nanofluids. In this paper, it is focused on this phenomenon by introducing a new application of image processing as a tool to study the morphological characteristics of micron sized clusters. Through this approach, it is demonstrated that higher thermal conductivity is obtained when particles micro-clusters are well percolated all over the base fluid. As a result, well-diffused percolating micro-structures are known to have a significant role in heat conduction of nanofluids. (C) 2016 Elsevier Ltd. All rights reserved.