Extended fins play a vital role in enhancing the thermal performance of many engineering applications. This paper studies the effect of using a condenser with semi-cylindrical longitudinal internal fins on the two-phase closed thermosyphon performance, experimentally. Six semi-cylindrical internal fins having 4mm radius circles are made along the internal part length in the condenser part of the thermosyphon. The internally finned thermosyphon is charged with water as a working fluid for 20%, 50% and 80% filling ratios. The experiments are performed for different input powers of 100, 150, 175, 200 and 250W. The best performance of finned thermosyphon occurred at a 50% fill ratio. Results showed that the use of semi-cylindrical internal fins reduced the thermal resistance for all of the filling ratios. A reliable enhancement of the overall heat transfer coefficient has been achieved. The thermal resistance of finned thermosyphon was also enhanced by 37% when compared with identical thermosyphon without fins under the same conditions. The worst performance occurs with semi-cylindrical fins with a 20% filling ratio.