In　this　paper,　the　authors　report　the　direct　growth　of　ZnO　nanowires　(NWs)　on　a　flexible　substrate　by　the　hydrothermal　process　and　the　fabrication　of　ZnO　NW-based　humidity　sensor.　It　was　found　that　average　length　and　diameter　of　the　ZnO　NWs　were　0.6　mu　m　and　50　nm,　respectively.　It　was　also　found　that　resistance　of　the　ZnO　NWs　decreased　by　45%　as　we　increased　the　relative　humidity　from　52%　to　90%.　Furthermore,　it　was　found　that　measured　resistance　was　very　stable　with　negligible　fluctuation　after　16　days　continuous　testing.