The flow boiling critical heat flux (CHF) of nanofluid in a vertical tube is affected by many influence factors, like type and concentration of nanofluid, pressure and mass flux. In this work, these influence factors was investigated experimentally. The results revealed that both AlN/H2O and Al2O3/H2O nanofluid could improve flow boiling CHF and enhancement rate could up to 18% maximally compared with deionized water. Nanofluid flow boiling CHF increases with increasing pressure, mass flux and diameter of test tube and decreases with increasing heating length of test tube. Meanwhile, the type and concentration of nanofluid has nearly no influence on CHF value. Furthermore, a mechanism which from the view point of nanoparticles deposition and adhering force of nanoparticles deposition layer was proposed to explain the CHF enhancement of nanofluid.