The remarkable characteristic of a two-phase flow is the pulsating phenomenon. In this study, the pulsation has been measured by an electrode probe as the pericdical fluctuation of the local void ratio to analyse the flow configuration quantitatively. The experiment has been carried out on the air-water two-phase vertical upward flow in a tube of 27.6 mm ID and 2595 mm length. This paper presents the period of the void ratio fluctuation, the pitch of the void ratio and the gas time ratio. And the empirical formulas of the average period and the average pitch are given. The behavior of the gas time ratio in this experiment agrees with that of the local void ratio which has been reported by other investigators. Thus, it is shown that the void ratio distribution reported before was the distribution of the time mean value of the local void ratio.