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In order to obtain the data on turbulent mixing rate between triangle tight lattice subchannels, which will be adopted as the next generation BWR fuel rod bundle, adiabatic experiments were conducted for single-and two-phase flows under hydrodynamic equilibrium flow conditions. The gas and liquid mixing rates measured for two-phase flows were found to be affected by the void fraction and/or flow regime, as reported in our previous study on a simulated square lattice rod bundle channel having hydraulic diameters of about four times larger than the present tight lattice channel. Comparing the present mixing rate data with those for the square lattice channel and a triangle one in other institution, we found that the mixing rate was considerably smaller in the present channel than the other ones, i.e., a channel size effect.