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Abstract: Boiling two-phase flows in a single-channel commercial brazing plate heat exchanger were visualized by thermal neutron radiography method, and the effect of flow direction, such as vertically upward or downward, on liquid distribution in the channel and boiling heat transfer performance was considered. The experiments had been carried out using thermal neutron radiography facility of JRR-3 of JAEA in Japan. The relationship between heat transfer coefficients and flow behaviors for three kinds of inlet condition, such as subcooled liquid, saturated liquid, and wet vapor, was investigated. Though vertically upward flow is generally selected for boiling flow to remove vapor bubble from heating surface by buoyancy force, it was shown from the result that downward boiling flow produced higher heat transfer performance than upward flow. Especially, the tendency was remarkable at low-quality condition. From the visualization results, it could be clearly observed that there was a large difference between upward and downward in-flow pattern around the inlet. The results show that the lowering of heat transfer performance was caused by the difference of flow pattern. [Copyright &y& Elsevier]