We introduced a metal-organic framework (MOF), HKUST-1, to a conjugated polymer layer, poly(3-hexylthiopene-2,5-diyl) (P3HT), and systematically investigated the effects of HKUST-1 in the P3HT active layer on the performances of fieldeffect transistor (FET) devices under humid conditions over 21 days. The performances of a P3HT film blended with HKUST-1 in an FET remained relatively stable under humid conditions over a long period of time. As the water molecules diffused into the bulk conjugated polymer layer, the HKUST-1 crystals in the P3HT film captured water molecules and prevented water adsorption by the P3HT backbones, suggesting an efficient approach to improving the environmental stability of polymer FETs. Furthermore, HKUST-1/P3HT blended films exposed to humidity displayed recovered device performances under vacuum. The device stability conveyed by the HKUST-1 in the presence of humidity was closely related to the superior ability of the HKUST-1 to capture water, which was based on the water adsorption characteristics of HKUST-1.