Abstract: The paper presents a method of measuring void fraction of steam generator by using optical fiber probes.Comparison is made between void fraction in a vertically upward tube by using optical fiber probes and the void fraction by quick-close valve.A new method of determining threshold is developed for signal processing from a synchronous measurement procedure.An experimental equation by which average void fraction can be obtained though measuring void fraction at axial center of tube.On-line measuring void fraction of steam generator with vertical tube can be achieved by using optical fiber probes.