The structural and dynamic properties of ethanol molecules in nanochannels of various diameters have been studied using molecular dynamics simulations. The properties of the ethanol molecules depended strongly on the nanochannel diameter. Single-walled carbon nanotubes (SWCNTs) were used to model the nanochannels. The average number of ethanol molecules inside a SWCNT increased monotonically with the SWCNT diameter. However, the average number of hydrogen bonds per ethanol molecule changed non-monotonically with the SWCNT diameter. The tube wall of the (6,6) SWCNT strongly constrained the ethanol molecules inside from rotating freely. As a result, an ethanol molecule within a (6,6) SWCNT formed more hydrogen bonds than one in a (7,7) or (8,8) SWCNT. These strong tube wall constraints also gave ethanol molecules in the (6,6) SWCNT the longest hydrogen bond lifetimes. More importantly, the diffusion rates of ethanol molecules inside a SWCNT first increased and then decreased as the SWCNT diameter increased. The non-monotonic dependence of the diffusion rate on the SWCNT diameter may be related to multiple factors, such as hydrogen-bonding properties, ethanol–tube interactions, reorientation properties, etc.