The stabilities of water molecules in carbon slit-shaped nanospaces have been studied using the potential calculation for possible water clusters (H2O)n) of n = 2-12. The adsorption isotherm of water on a graphite slit pore (w = 1.1 nm) with no surface functional groups at 303 K was calculated with GCMC simulation using TIP-5P and 10-4-3 Steele potential functions; this simulated isotherm has a vertical uptake at P/P-0 = 0.5. The cluster growth along the vertical adsorption uptake was evidenced through the snapshot of GCMC simulation. The simulated adsorption isotherm agreed well with the experimental isotherm of water on an activated carbon fiber (ACF) having uniform slit pores. Thus, H2O molecules are adsorbed in hydrophobic carbon nanopores without surface functional groups through cluster formation. The isosteric heat of adsorption of clusters of (H2O)(n=8-10) obtained from the GCMC simulation coincided well with the experimental value. The radial distribution function of the clusters from the GCMC simulation is close to the structure of ice I-h. Therefore water molecules can gain an explicit hydrophobicity through clusterization in nanopores.