An experimental study has been made to elucidate an effect of oscillated flow induced near critical heat flux (CHF) and natural circulated flow of vapor and liquid in a vertical tube on the CHF. The experiment has been carried out for the condition of heated tube length of L = 100-1000 mm, tube diameter of D = 4-9 mm and the CHF is measured under the condition that the exit of the unheated tube connecting the heated tube is extruded into a vapor chamber to prevent a liquid flowing into the heated tube from the top. The experiment reveals that the CHF for D = 7 and 9 mm and any tube length from 100 to 1000 mm becomes identical to that for the case of the tube top in the liquid, while the CHF for D = 4 and 5 mm is smaller than that for the case of the tube top in the liquid due to no liquid supply from the top. It is found that the oscillation induced near the CHF increases the CHF.