<p>The effects of cross-sectional shapes of flat-plate pulsating heat pipes (PHPs) on heat-transfer performance were experimentally investigated. The flat-plate PHP, made of aluminum alloy, has a closed-end serpentine channel with 22 turns. Four PHP samples with different channel shape (square or circular), the hydraulic diameter of the channel (1 or 1.2 mm), and plate thickness (3 or 2 mm) were prepared. The working fluid in the channel is R1336mzz(Z), and its filling ratio was increased from 30 mass% to 70 mass% in increments of 10 mass%. The heating section of the PHP is heated at power from 20 to 180 W in one of three heating orientations (vertical bottom, top, and horizontal) and the cooling section of the PHP is kept at 40 °C. Equivalent thermal conductivity is calculated from temperature on the surface of the PHP and heat transport rate. The experimental results indicate that the larger the cross-sectional area of the channel-wall material, the more likely it is that working-fluid oscillation will occur in the case of the top and horizontal heating orientations. Within the scope of this study, The PHP sample with a 1.2×1.2-mm square channel, the thickness of 3 mm, and filling ratio of 40 mass% has the highest equivalent thermal conductivity, namely, 2880 W/(m･K) at heat-transport rate Q of 146 W in top-heating orientation, 2750 W/(m･K) at Q = 146 W in horizontal-heating orientation, and 6540 W/(m･K) at Q = 36 W in bottom-heating orientation. As for that PHP, when Q is 80 W or more, equivalent thermal conductivities are the same regardless of heating orientation. It is thus considered that under that condition, working-fluid behaviors of the PHP become equivalent regardless of the heating orientation.</p>