Critical heat flux (CHF) experiments are conducted to investigate the flow boiling CHF enhancement by additives and a heater material for a wide range of exit qualities and void fractions and for pressure conditions of 1 and 2 bar using the rectangular curved flow channel. For the wettability improvement to enhance the CHF, the additives of boric acid and tri-sodium phosphate (TSP, Na3PO4 12H2O) and the heater material of low alloy steel (SA508) are used. The trend of the CHF enhancement in terms of the flow conditions is discussed for the slug flow regime. The CHF is observed in the bubbly-slug and slug flow through flow visualization works. For each condition of pressure, the CHF is enhanced in most of the experimental cases under relatively high exit quality condition due to the improvement effect of surface wettability. The CHF is not enhanced under the relatively low exit quality condition, because the sufficient liquid supply outplays the difference of wettability. The relationship between CHF enhancement ratio and exit quality, which is observed in each pressure condition, is not effective in comparison with those for the different pressure condition. The void fraction is additionally considered to discuss the CHF enhancement according to void condition. The CHF enhancement ratio is clearly varied by the void fraction for all pressure conditions. The CHF enhancement ratio is proportional to the void fraction for the void fraction of ∼0.5 and above, and it is close to 1 in the void fraction of ∼0.5.