The microscopic magnetic structure of a vortex in a Nb single crystal was i
nvestigated using small-angle neutron scattering by measuring higher order
reflections. By fitting-the experimental scattering intensities, the magnet
ic structure of a vortex can be represented by the London equation with an
additional Gaussian factor due to the thermal displacement of vortices. The
radius of a vortex (20.3 nm at 3.3 K) is somewhat smaller than the London
penetration depth (lambda (L) (0) = 31.5-39.0 nm).