This letter presents fabrication of a flexible 1.5-μm -channel-length silicon thin-film transistor (TFT) on a plastic substrate with a cutoff frequency f_T of ~ 3.7 GHz and a maximum oscillation frequency f_max of ~ 12 GHz. Radio-frequency (RF) characterization is conducted for the flexible TFT under uniaxial mechanical bending conditions, indicating slight but notable monotonic performance enhancement with larger bending strains. Equivalent circuit model and theoretical analysis are employed to understand the underlying mechanism. Flexible gigahertz TFTs are shown to be naturally suitable for high-performance RF/microwave applications under mechanical bending (deformation) environment. This letter provides insight on designing and employing flexible gigahertz active devices.