As a preeminent working fluid, nanofluid has been attracted great attention since it was proposed based on nanotechnology. Flow boiling is familiar in heat transfer system and the critical heat flux is a key parameter for the design of thermal hydraulic. However, the research on the critical heat flux of nanofluid flow boiling is insufficient and no correlation is proposed for nanofluid flow boiling. Thus, the critical heat flux of nanofluid flow boiling is experimentally investigated in a vertical tube with the consideration of outlet pressure, mass flux, inlet subcooling, heating length and diameter, nanoparticle type and concentration. The results indicate that the critical heat flux of nanofluid flow boiling is enhanced (up to 18% in present work) compared with base fluid and the increasing radio is increased with increasing the mass flux, diameter and pressure, and with decreasing the heating length. In addition, the inlet subcooling, nanoparticle types (Al2O3, AlN) and concentrations (0.1 vol%, 0.5 vol%) have almost no significant influence. Moreover, the increasing is explained from the view point of adhering of liquid sublayer and deposition of nanoparticles. Furthermore, a correlation of nanofluid flow boiling critical heat flux is proposed by the experimental results with excellent accuracy and applicability. (C) 2018 Elsevier Ltd. All rights reserved.