The effect of using silver nanofluid (de ionized water mixed with silver nano and particles less than 100 nm) on the thermal characteristics of a two-phase closed thermosyphon in an operating state was investigated in this research. The thermosyphon was made with copper tubes with 7.5, 11.1 and 25.4 mm ID. The filling ratios of 30%, 50% and 80% by evaporator length and aspect ratios of 5, 10, and 20 with an inclination angle 90 degrees. The operating temperatures were 40 degrees C, 50 degrees C and 60 degrees C. The research reports the effect of dimensionless parameters on heat-transfer characteristics were: L/d(i), Bo, Pr, We, Fr, Ja, Co, Nu, Ar, Gr and Ku. It was found that the filling ratio has no effect on the ratio of heat-transfer characteristics in the vertical position, but the properties of the working fluid affected the heat-transfer rate. In addition a correlation for predicting the heat flux for the two-phase closed thermosyphon in the vertical position has been established. (C) 2010 Elsevier Ltd. All rights reserved.