Abstract: Boiling heat transfer of flowing high Prandtl number fluids was studied experimentally in different types oi thermosyphon boiling device with various geometrical dimensions. The working media were ethanol, acetone and iso-butanol. The range oi Prandtl number was from 3.81 to 26.3. Experimental results were compared with predicted values calculated by different correlations from literature. The influence ot Prandtl number on two-phase flow heat transfer was in terms oi its mechanism. The correlations of two-phase flow factor, Ftp were examined and discussed.