Vertical straight wickless-heat pipe (VSWHP) is now being chosen as one of the most efficient and inexpensive passive cooling system technology. One potential application of VSWHP is to remove the heat generation in a water cooling tank under accident condition in small modular reactor. The objective of this investigation is to know the influence of filling ratio on the overshoot phenomenon of VSWHP with low temperature source. A series of experimental investigation on vertical straight wickless-heat pipe was carried out with variation of working fluid filling ratio charged in to VSWHP of 60, 80, and 100%, with constant value of temperature source of 60°C and coolant volumetric flow rate of 8 L/min. The experiment results obtained show that, with lower temperature source, filling ratio of 60% resulted faster overshoot phenomenon inside the vertical straight wickless-heat pipe. The ideal amount of filling ratio charged in the heat pipe depends on the temperature source in the evaporator section. For low temperature source, the lower filling ratio will accelerate the occurrence of the overshoot phenomenon and natural circulation in the VSWHP.