A high-performance flexible pulsating heat pipe (FPHP) hermetically sealed with metal has been developed as a flexible heat spreader. For this, experimental works are conducted to evaluate the thermal performance and long-term reliability of the FPHP. The FPHP consisted of a polycarbonate sheet and flexible-copper-clad-laminates (FCCLs). Rectangular channels with dual hydraulic diameters of 0.75 mm and 0.5 mm were engraved on the polycarbonate sheet. The width, length, and thickness of the FPHP were 53.4 mm, 85.5 mm, and 0.64 mm, respectively. To prevent the permeation of non-condensable gases (NGCs) from the ambient, the upper and lower sides of the channel wall were encapsulated with the copper side of the FCCLs by a silane-mediated bonding method, and the flanges of the FPHP were sealed by soldering the two copper-foils. As the working fluid, HFE-7000 was charged into the FPHP. The FPHP has a maximum effective thermal conductivity of 1070 W/m.K, which is approximately 2.7 times higher than that of copper in the vertical orientation. The effective thermal conductivity of the FPHP decreases by 20.4% and 3.4% for the horizontal orientation and 45-degree bent condition, respectively. The long-term reliability of the FPHP was evaluated by measuring its gas permeability. The results show that the rate of the pressure increase inside the FPHP is approximately 0.54 Pa/day, indicating that the lifetime of the FPHP is at least 2000 days. The proposed method enables the fabrication of a thin and flexible pulsating heat pipe with high thermal performance and guaranteed long-term reliability.