Vertical ZnO nanowires with a very high aspect ratio of more than 250 were synthesized by thermal chemical vapor deposition (CVD) on vertical nanowires grown by hydrothermal growth (HG) without using the metal catalyst. A ZnO seed layer grown by sputtering was crucial to synthesize the vertical nanowires in both the CVD and the HG processes. The underlying vertical nanowires grown by the HG process (HG-nanowire) enhanced nucleation of ZnO during the followed ZnO synthesis by CVD. Hydrogen-gas sensitivity was measured from the synthesized ZnO nanowires. High sensitivity of 72% and response time as fast as 30 s was observed at 200 degrees C from the vertical ZnO nanowires grown by CVD on the HG-nanowire, which is much higher than the sensitivity value of 50% from the vertical HG-nanowire and the value of 35% from the vertical CVD nanowires without the HG-nanowire. The results indicate that the method synthesizing the vertical ZnO nanowires by combining the HG process and thermal CVD is a very promising way to fabricate both the vertical nanowires and the highly sensitive gas sensors.