Abstract: This paper presented an experiment on critical heat flux（CHF） of vertical up smooth tube in the near-critical pressure region. The operating pressure ranges from 18 22 MPa, mass flux ranges from 310~550 kg·m-2·s-1, inlet subcooling temperatures range from 5 to 10℃. The diameter and thickness of the test section are 30 mm and 5.5 mm. The wall temperature distributions are obtained. The effects of parameters on heat transfer characteristic were analyzed. The heat transfer coefficients of normal heat transfer and heat transfer deterioration were derived. The correlation of CHF was derived and compared with four classical correlations. The effects of parameters on CHF were analyzed. The reasons of heat transfer deterioration and diameters of the bubbles were analyzed. The results show that the heat transfer deterioration occurs earlier when the pressure is more closed to the critical pressure or the mass flux is smaller. The cross sections is lower, the vapor quality during heat transfer deterioration is smaller. The heat transfer deterioration of this experiment is mainly departure from nucleate boiling（DNB）.