We have used a sol-gel spin-coating process to fabricate a new metal-insulator-metal (MIM) capacitor comprising a 10 nm-thick high-k thin dielectric HfO 2 film on a flexible polyimide (PI) substrate. The surface morphology of this HfO 2 film was investigated using atomic force microscopy and scanning electron microscopy, which confirmed that continuous and crack-free film growth had occurred on the film surface. After oxygen (O 2 ) plasma pretreatment and subsequent annealing at 250 °C, the film on the PI substrate exhibited a low leakage current density of 3.64 × 10 -9 A cm -2 at 5 V and a maximum capacitance density of 10.35 fF μm -2 at 1 MHz. The as-deposited sol-gel film was completely oxidized when employing O 2 plasma at a relatively low temperature (ca. 250 °C), thereby enhancing the electrical performance. We employed X-ray photoelectron spectroscopy (XPS) at both high and low resolution to examine the chemical composition of the film subjected to various treatment conditions. The shift of the XPS peaks towards higher binding energy, revealed that O 2 plasma treatment was the most effective process for the complete oxidation of hafnium atoms at low temperature. A study of the insulator properties indicated the excellent bendability of our MIM capacitor; the flexible PI substrate could be bent up to 10 5 times and folded to near 360° without any deterioration in its electrical performance.