Effects of flow fluctuation at the test section inlet on flow boiling heat transfer were investigated for FC72 by the use of a horizontal circular tube with a diameter of 0.51mm. Flow fluctuation was minimized by employing a high-powered syringe pump in one experiment, while it was intensified in another experiment by the connection of an auxiliary tank exposed to atmosphere allowing the reverse flow. In the experiments of strict inlet flow rate regulation, heat transfer characteristics were similar to those observed in normal size tubes. But if the flow rate was fluctuated up to ±20% of the total under the weak inlet flow rate regulation, the trend that the heat transfer coefficient was increased with increasing vapor quality in moderate vapor quality region was disappeared and heat transfer deterioration due to partial dryout started at lower vapor quality. Boiling heat transfer characteristics in minichannels could be changed considerably by the existence of flow fluctuation caused by the rapid axial growth of elongated bubbles at low vapor quality. The regulation of inlet flow rate seems to be one of key parameters to reduce the scattering in heat transfer data encountered in flow boiling of minichannels. Experimental data obtained by using a pump of weak power or data using a liquid reservoir can never be inherently consistent with those obtained for the constant inlet flow rate conditions. Although such weak restriction of inlet conditions are actually encountered in application systems, the difference in heat transfer characteristics between the normal and mini tubes should be clarified, as the first objective of the research, under the same inlet conditions without flow fluctuation.