Small angle X-ray scattering (SAXS) is measured for the lamellar phase in aqueous systems of 1-o-β-3,7-dimethyoctyl-d-glucopyranoside (β-Glc(Ger)), which has recently been prepared by us, 1-o-β-decyl-d-glucopyranoside (β-GlcC 10 ), and 1-o-β-octyl-d-glucopyranoside (β-GlcC 8 ). The repeat distance d obtained from the position of the diffraction peak does not follow the swelling law d = 2δ hc /ϕ hc , where δ hc and ϕ hc are the thickness and the volume fraction of the hydrophobic layer, respectively. This may result from the fact that δ hc increases and, equivalently, the surface area per surfactant molecule (a s ) decreases with increasing concentration. So we calculate δ hc and a s from the observed d value at each concentration using the above swelling law. The half-thickness δ hc increases in the order β-GlcC 8 < β-Glc(Ger) < β-GlcC 10 at a fixed concentration. On the other hand, the data on a s for β-GlcC 10 and β-GlcC 8 lie on the same line and the data for β-Glc(Ger) lies above this line. These results suggest that the cross-sectional area of the geranyl chain is larger than that of the glucose headgroup. Existence of water filled defects in bilayer sheets is also discussed based on the SAXS pattern and the concentration dependence of d.