Experiments have been performed to observe flow pattern and investigate convective heat transfer for air-water vertical flow in a forced circulation system. The bubbles motion was recorded by a high-speed CCD camera. For the bubble and slug flow pattern, the temperature fluctuation signals and local heat transfer coefficients were obtained in a short heated tube. The probability density function classical was applied to analyze the temperature. The results qualitatively reflected characteristics of local heat transfer in two phase flow comparing with that in single-phase liquid. The comparison revealed that the gas phase could enhance the heat transfer.