This study is focused on the comparison of heat transfer performance of two thermosyphons having 60 straight and helical internal grooves. Distilled water has been used as working fluid. Liquid fill charge ratio defined by the ratio of working fluid volume to total internal volume of thermosyphon, the inclination angle and operating temperature were used as experimental parameters. The heat flux and heat transfer coefficient are estimated from experimental results. The conclusions of this study may be summarized as follows; Liquid fill charge ratio, inclination angle and geometric shape of grooves were very important factors for the operation of thermosyphon. The optimum liquid fill charge ratio for the best heat flux were $30\%$. The heat transfer performance of helically grooved tube was higher than that of straight grooved tube in low inclination angle (less than $30^{circ}$), but the results were opposite in high inclination angle (more than $30^{circ}$). As far as optimum inclination angle concerns, range of $25^{circ}~30^{circ}$ for a helically grooved tube and about $40^{circ}$ for a straight grooved tube are suggested angles for the best results.