The bubble generation process in a microfluidic T-junction is investigated by high-speed observation to develop a novel technique for monodispersed microbubble generation. The μ-PTV (micron-resolution Particle Tracking Velocimetry) is operated to measure the flow field on the bubble generation by seeding 1.0 urn particles wtih bright-field microscopy. The bubble generation process is highly periodic; therefore, μ-PTV is iteratively con- ducted in the same phase. Time-series velocity-vectors at the order of 1 m/s are measured by this high-speed μ-PTV method. The experimental results show that the proposed technique enables generation of 20〜70 μm diameterbubbles at frequency of 1〜10^2 kHz. Furthermore, the high-speed observation indicates that the bubble generation consists of two stages; intruding stage and squeezing stage. According to the obtained flow field, the liquid gradu- ally flows into the side channel with the growth of the gas tip in the main channel.