An experimental study of bubble distribution at various flow regimes has been carried out in air-water two-phase flows flowing upward in a 40mm diameter acrylic tube under atmospheric pressure. To obtain local void fraction, U-shaped optical probe is used. The probe signal is amplified by electric circuit. The amplified signal is sampled uniformly using the analog to digital converter and stored and processed in IBM-PC/AT computer. The probe was very sensitive to the existence of air and water phase and can be used to obtain not only local void fraction but also other interesting parameters. In bubbly and slug flow regimes, the signals of the probe are different and this difference in the signal shape may be used as an indirect indication of the flow pattern transition. In this study, local time-averaged void fraction is measured and bubble behavior is observed at various flow rates. Results show that in bubbly flow, wall peaks of void fraction occur near tube wall and in slug flow, void fraction is maximum at tube center.