Extended fins play an important role in improving the thermal performance of many engineering applications. This manuscript studies the effect of hemi circular cross sectional longitudinal internal fins on the wickless heat pipe performance. The experimentally studied along the length of the internal section in the thermosyphon's condenser part, six hemi circular cross-section extended internal fins with a 4 mm radius circle are formed. Water is used as the working fluid in the internally finned thermosyphon at filling ratios of 20%, 50%, and 80%. Experiments are conducted at different powers: 100, 150, 175, 200, and 250 watts. The results showed that using hemispherical internal fins decreased the thermal resistance for filling ratio of 50%. The thermal resistance of finned TPCT also enhanced by 39% at low heat input. The worst TPCT with semicircular fins performance occurred at 20% fill ratio due to lake of fluid.The worst thermal performance occurred at filling ratio of 20% and power input level of 100 watt.