Dryout characteristics of forced convective water flow in a vertical straight tube are experimentally studied in order to evaluate the practical performance and design of a LMFBR (Liquid Metal Fast Breeder Reactor) steam generator. Experiments were carried out over the range of mass velocity 160~550 kg/m²s and steam pressure 18~153 kg/cm², employing two different methods of heating such as sodium-heating and joule-heating. First, a new correlation is proposed for the dryout quality in terms of steam pressure and mass velocity. Second, the characteristics of the wall temperature around the dryout point, such as the temperature gradient along tube axis and the oscillatory behavior, are studied in relation to the steam pressure and mass velocity.