Interfacial importance and nanopore structures of single wall nanocarbons such as single wall carbon nanotube (SWCNT) and single wall carbon nanohorn (SWCNH) are described in terms of intermolecular interaction theory. The relationships between the nanopore structure and nanoconfinement effect for molecules and ions are shown using examples on supercritical H2 adsorption on SWCNT, quantum molecular sieving effect of SWCNT for H2 and D2 below 77 K, water structure and hydration structure of Rb, Cs, and Sr ions in nanopores of activated carbon fiber and SWCNH, and capacitance storage of SWCNH. The SWCNT produced by laser ablation shows upward-curved adsorption isotherm of H2 at 77 K, suggesting the presence of the strong sites. The adsorption amount of D2 on this SWCNT at 77 K was clearly larger than that of H2 by about 20% at 77 K due to quantum effect. The hydration number around Cs and Sr ions in 1.1 nm slit pores of ACF reduced by more than 20 %. The capacitance depended on the opening of SWCNH and charging time for organic electrolyte (C2H 5)4NBF4 in propyrene carbonate.