Wettability effects of nanoparticles on pool boiling heat transfer of a nanofluid are investigated experimentally for the first time in this paper. The mechanism on wettability effects of nanoparticles in pool boiling is revealed by a visualization study of bubble dynamics in the nanofluid, and by analyzing the morphology of nanoparticle deposition layers on the heater surface as well as residue patterns after nanofluid droplets evaporation on a glass surface. The enhancement and deterioration of boiling heat transfer of nanofluids as compared with their base fluid are attributed to different wettabilities of nanoparticles, resulting in different bubble behaviors and different morphologies of nanoparticle deposition layers. It is found that (i) nanofluids containing moderately hydrophilic nanoparticles enhance boiling heat transfer at fully-developed nucleate boiling regime with a significant increase in critical heat flux, and (ii) nanofluids containing strongly hydrophilic nanoparticles do not enhance nucleate boiling heat transfer, and the strongly hydrophilic nanoparticle deposition layer deteriorates boiling heat transfer coefficient. (C) 2016 Elsevier Ltd. All rights reserved.