Nanoparticle suspension fluids, called nanofluids, have attracted much attention. It is not straightforward to predict effective thermal conductivity of the nanoparticle suspension fluids. One of that reasons is that aggregation may transform particle shape and diameter, and cause particle sedimentation in the fluid. In the present study, fine carbon particles of graphite and carbon black were mixed with surfactant-added water. The fine carbon particles were nanoscale in the primary particle diameter. Time variation of thermal conductivity of the fine-particle suspension fluids was evaluated with transient hot-wire method. In addition, zeta potential of the fine particles, the aggregation diameter and degree of the sedimentation in the fluids were evaluated. In the present study, it turned out that thermal conductivity of the graphite particle suspension decreased as the particle sedimentation proceeded, and it was characterized by Hamilton-Crosser model. Moreover, we found that thermal conductivity of the carbon black suspension decreased with time due to sedimentation, and was lower than that of the graphite suspension. The present experimental results indicated that the difference between the graphite and carbon black suspensions arose from the functional groups formed on the particles surface that determined particle surface states and influenced the particle aggregation. (C) 2018 Elsevier Ltd. All rights reserved.