The paper describes the effect of fluid's property on the heat transfer characteristics of the inclined thermosyphon (0-90 deg.) by taking into account of the filling ratio. Stainless steel thermosyphon is used and three kinds of working fluid, which are ethanol, freon (R-113) and Fluorinert (FC-72), are selected as the working fluid. The chosen filling ratios are 80%, 50% tmd 30%. The critical heat transfer rate, Qcritical, and the thermal resistance at any inclination angle are determined in the angles' range of the isothermal operation. With the experiments taken at very near to the boiling point, the ratio of Qcritical at any inclination angle to the Qcritical at 90 deg. are determined as the function of the inclination angle. It is found from the experiments that there is a tendency for all of the working fluid that the ratio of Qcritical/Qc90 at any inclination angle is higher than that of the vertical position, and the effect of inclination angle can be classified into 4 major regions (vertical position - about 70 deg., 70 deg. - about 50 deg., 50 deg. - minimum angle of isothermal operation, and minimum angle - limiting angle of operation). The maximum ratio of Qcritical/Qc90 which is obtained to be 1.8 in the case of FC-72 with 50% filling ratio, depends on the property of the working fluid. The larger the latent heat of vaporization, the lower the maximum value of Qcritical/Q. The effect of filling ratio on the range of isothermal operation (from 90 deg. to the minimum angle of isothermal operation) can be obviously seen in the case of R-113 and FC-72. For ethanol, although the effect of filling ratio on the range of isothermal operation can be seen in the case of small filling ratio, the effect of filling ratio on Qcritcal/Qc90 can be neglected.