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This paper reports a modified polyimide and its application to a microwave relative humidity (RH) sensor. For a highly sensitive moisture sensing layer, a polyimide was synthesized from oxidianiline (ODA), m-pyromellitic dianhydride (m-PMDA) and N-methylpyrrolidone (NMP) instead of using a commercial polyimide. The synthesized polyimide was applied to a microwave humidity sensor, CPW-to-slotline ring resonator in this case. Characteristics of the fabricated microwave humidity sensor including sensitivity, hysteresis and stability were analyzed for various relative humidity levels. Sensitivity of the RH sensor, in terms of resonant frequency change, was 181 kHz/%RH, hysteresis was 0.013%, and deviation from an average resonant frequency was 0.002% at 40%RH.