One cavitation structure in which the channel cross section expanded suddenly was introduced in single straight microchannel. The experiment was carried out with R-134a as the fluid medium, which was driven by a gear pump. The flow pattern was observed by a charged coupled device (CCD) camera and microscope. The average boiling heat transfer coefficient was estimated with the calculation method proposed in this paper. The experimental results show that the boiling began at the cavitation structure, and stable flow boiling was maintained. When heat power rose, the boiling became strong; then the pressure drop in the micro-channel increased and heat transfer was enhanced. The liquid percentage in two-phase flow increased and the length of boiling area became small when the liquid subcooling degree rose with the fixed heat power.