Organic-inorganic hybrid dielectrics have attracted considerable attention for improving both the dielectric constant (k) and mechanical flexibility of the gate dielectric layer for emerging flexible and wearable electronics. However, conventional solution based hybrid materials, such as nanocomposite and self-assembled 1 nanodielectrics, have limitations in the dielectric quality when the thickness is deep-scaled, which is critical to realizing high-performance flexible devices. This study proposes a novel vapor-phase synthesis method to form an ultrathin, homogeneous, high-k organic-inorganic hybrid dielectric. A series of hybrid dielectrics is synthesized via initiated chemical vapor deposition (iCVD) in a one-step manner, where 2-hydroxyethyl methacrylate and trimethylaluminum are used as the monomer and inorganic precursor, respectively. The thickness and composition are effectively controlled to form a uniform, defect-free hybrid dielectric. As a result, the synthesized hybrid dielectric has a high-k value as high as 7 and exhibits a low leakage current density of less than 3 X 10(-7) A/cm(2) at 2 MV/cm, even with an equivalent oxide thickness of less than 5 nm. Furthermore, the dielectric layer shows exceptional chemical stability without any degradation in its dielectric performance and a smooth surface morphology. The dielectric layer also has good flexibility, maintaining its excellent dielectric performance under a tensile strain of up to 2.6%. Organic thin-film transistors with the developed hybrid dielectric as the gate dielectric achieved hysteresis-free transfer characteristics, with an operating voltage of up to 4 V and excellent mechanical flexibility as well. The hybrid dielectric synthesized via the iCVD process is a promising candidate for high-performance, low-power flexible electronics.