Abstract: The factors with effects on the sub-cooled boiling heat transfer are vapor quality and system pressure. System pressure is with direct effects on the bubble size of the heated surface, and the heat transfer from heated surface to liquid. The thermodynamic quality in sub-cooled boiling region is calculated based on the thermodynamic parameters and flow parameters, and then the void fraction at certain point is calculated. The calculation correlations for the void fraction in sub-cooled boiling channel are given. It is proved that the correlations coincide with the experimental dada within the given experimental parameters.