In this paper, we report the design, fabrication and measurement results of a highly sensitive capacitive-type humidity sensor using a customized polyimide film without hydrophobic elements for evaluating the hermeticity of micropackages. For higher sensitivity, a customized polyimide film as a moisture-absorbing layer is applied instead of using general polyimide films for microelectronic applications. The polyimide film is obtained by synthesizing polyamic acid composed of m-pyromellitic dianhydride, phenylenediamine and dimethylacetamide followed by thermal polymerization. An assembly of the humidity sensor and a micropackage which is realized with localized heating and bonding with a polysilicon heater is also fabricated to verify packaging compatibility. Characteristics of the sensor which include sensitivity, hysteresis and stability are measured. The measurement results show a percent normalized capacitance change of 37.45/%RH, hysteresis of 0.77% over a range from 10 to 90%RH and stability of 0.25% maximum drift from the average value at 50%RH for 120 min. According to these results, it is expected that the proposed humidity sensor can be applied to evaluate the hermeticity of micropackages.