We demonstrate a new flexible metal-insulator-metal capacitor using 9.5-nm-thick ZrO2 film on a plastic polyimide substrate based on a simple and low-cost sol-gel precursor spin-coating process. The surface morphology of the ZrO2 film was investigated using scan electron microscope and atomic force microscope. The as-deposited ZrO2 film under suitable treatment of oxygen (O2) plasma and then subsequent annealing at 250 °C exhibits superior low leakage current density of 9.0 × 10-9 A/cm2 at applied voltage of 5 V and maximum capacitance density of 13.3 fF/μm2 at 1 MHz. The as-deposited sol-gel film was completely oxidized when we employed O2 plasma at relatively low temperature and power (30 W), hence enhancing the electrical performance of the capacitor. The shift (Zr 3d from 184.1 eV to 184.64 eV) in X-ray photoelectron spectroscopy of the binding energy of the electrons towards higher binding energy; clearly indicates that the O2 plasma reaction was most effective process for the complete oxidation of the sol-gel precursor at relatively low processing temperature.