This paper presents a method to design 3D flexible antennas using liquid metal and additive technology (3D printer based on Fused Deposition Modeling (FDM) technology). The fabricated antennas present flexible properties. The design method is first presented and validated using the example of a simple inverted F antenna (IFA) in Ultra High Frequency (UHF) band. The design, the fabrication and the obtained measured results are discussed.