Millimeter-tall vertically aligned single-walled carbon nanotubes (SWCNTs) were grown in 10−15 min by chemical vapor deposition from C₂H₂/Ar with or without water addition using Fe catalyst supported on an Al−Si−O underlayer. Using combinatorial catalyst libraries coupled with the real-time monitoring of SWCNT growth, the catalyst and chemical vapor deposition conditions were systematically examined, and millimeter-tall SWCNTs were obtained even without water addition. The key for millimeter-scale growth of SWCNTs is to limit the C₂H₂ supply to below a certain partial pressure to retain an active catalyst. Water prolongs the catalyst lifetime under excess C₂H₂ supply, whereas it deactivates small catalyst particles and degrades the quality of SWCNTs at the same time. We also observed a gradual increase in the diameter of SWCNTs with growth because of the coarsening of catalyst particles and found that water had no effect on this phenomenon. We demonstrate millimeter-tall SWCNTs grown by simply using C₂H₂/Ar gas without water addition, which revealed the mysterious role of water, and we show a practical route for the large-scale production of SWCNTs.