In this study, the critical heat flux (CHF) and heat-transfer coefficient under the pool-boiling state were tested using multi-wall carbon nanotubes (MWCNT) CM-95, CM-100, and oxidized MWCNT CM-100. The results showed that the highest CHF increase for both MWCNT CM-95 and CM-100 was at the volume fraction of 0.001%, and that the CHF increase ratio for MWCNT CM-100 nanofluid with long particles was higher than that for MWCNT CM-95 nanofluid with short particles. In addition, at the volume fraction of 0.001%, the oxidized MWCNT CM-100 nanofluid indicated a 47.27% higher CHF-increase ratio as well as an approximately 21.04% higher heat-transfer coefficient increase ratio compared with the MWCNT CM-100 nanofluid without oxidation treatment. Copyright (c) 2015 John Wiley & Sons, Ltd.