Accurate measurement of two-phase flow parameters is essential to evaluate and enhance safety of components for nuclear power plant. The purpose of this study is to develop double optical fiber sensor method for an accurate measuring of local bubble parameters such as bubble interfacial velocity, bubble diameter and so on. Since the geometry of the double optical fiber sensor affects the accuracy of the measured parameters, Monte-carlo simulation was carried out to determine the optimal design of the double optical fiber sensor. The double optical fiber sensor manufactured according to the optimal design was applied to the measurement of bubble parameters in the vertical square channel under the air-water flow condition. The performance of the sensor was verified by comparing the measured bubble parameters with those obtained by visualization method.