Nanofluid is employed as working medium for conventional 211µm width×217µm depth grooved circular heat pipe. The nanofluid used in this study is an aqueous solution of 10nm and 35nm diameter silver nanoparticles. The experiment was performed to measure temperature distribution and compare thermal resistance of heat pipe with nanofluid and with DI-water. The tested concentrations of nanoparticles range from 1mg/l to 100mg/l. The condenser section of the heat pipe was attached to a heat sink, which was cooled by water supplied from a constant temperature bath maintained at 40℃. At a same charge volume, the measured temperature distribution of nanofluid filled heat pipe have demonstrated that the thermal resistance decreases 30%~75% compare to DI-water case at input power of 30~60W. The measured results also show that the thermal resistances of heat pipe decrease as an increase of the size and the concentrations of silver nanoparticles.