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We have achieved the density control of carbon nanotubes (CNTs) by changing ethanol vapor flow speed in chemical vapor deposition (CVD). We calculated the ethanol mean free path under the CVD condition. From the calculation we estimated the minimum width of the flow channel that ethanol molecules can act as continuum. We fabricated the flow channels on a silicon wafer where ethanol vapor runs with its speed changing. Holes which CNTs can bridge were fabricated at the bottom of the flow channels. After the CVD process, we observed CNTs bridging the holes with scanning electron microscope (SEM). As a result, we observed that density of the synthesized CNTs, in other words, total length of the CNTs bridging the holes, increased with increasing the ethanol vapor flow speed.